Fed up with brokers who are making unsolicited offers for the neighborhood’s modest wood-frame houses, homeowners on the south side of Park Slope are placing signs in their windows that announce: “House Not For Sale.”

The 18-by-24-inch signs are the brainchild of a graphic designer, 34-year-old Aaron Brashear, who moved into a house on 23rd Street across from Green-Wood Cemetery last September, after purchasing the home for $345,000. One month later, when Mr. Brashear and his wife, Mic Holwin, 44, applied for a home equity line of credit, their house was reappraised for $550,000.

The three-dozen blocks from 18th Street to 25th Street between Fourth and Prospect avenues are dubbed “Greenwood Heights” by brokers, but lifelong residents of the neighborhood, such as Veronica Morciglio, 42, scoff at the moniker, preferring to call their neighborhood “South Park Slope.”The area used to be predominantly Italian and Polish, and its working-class character contrasted starkly with the brownstone-studded neighborhood to the north. But now, the neighborhood is becoming increasingly ethnically diverse, as bargain-hunters priced out of the borough’s costlier sections move in.

Still, an agent at Brenton Realty, Brian Graham, said that most of the firm’s customers looking to buy homes in Greenwood Heights are “not really investors, but the guy who works for the local accounting firm or a plumber with a family of three.”

In the last three years,housing prices in Greenwood Heights have shot up faster than in any other area abutting Park Slope, according to a partner at Brenton Realty,Tammy Shaw. The same three-family frame house on 21st Street that sold for $515,000 in July 2004 closed for $650,000 this past April, according to Ms. Shaw.

Greenwood Heights is an R-6 area under city zoning laws, allowing developers to build apartments as tall as a dozen stories in the neighborhood. Mr. Brashear and Ms. Holwin are leaders of a citizens’ group fighting to “downzone” the neighborhood in order to preclude high-rise construction. The 250 “Not For Sale” signs that Mr. Brashear has printed and distributed for free to community members feature two lines of black text that read: “This home supports responsible & contextual development in our neighborhoods.”

“When you have 20 houses on a block and 10 of them have ‘Not For Sale’ signs up, it sends a real message to realtors and developers,” Mr. Brashear said.

Local developers are undeterred. A partner at First Deal Realty, Jonathan Falcone, who has purchased four properties on Mr. Brashear’s block, said that he does not pay attention to the red-white-and-black signs that are spreading like wildfire across western Brooklyn.

One house on 23rd Street that had prominently displayed a “Not For Sale” sign nonetheless recently went on the market, Mr. Falcone said.

Standing amid 200 tombstones in the showroom of Supreme Memorials, 25-year-old Joseph Cassara said that despite the “Not For Sale” sign on his storefront, he still receives offers for the shop’s property on Fourth Avenue. Mr. Cassara’s great-grandfather bought the 60-by-80-foot lot for $40,000 in the early 1960s. Today, bids for the shop’s property routinely run in excess of $2 million, Mr. Cassara said.

But Mr. Graham of Brenton Realty said that the signs are “totally helpful” for his firm because they allow realtors to avoid sending wasteful fliers to homeowners who have no interest in selling their properties. And Ms. Morciglio said that not a single prospective home-buyer has knocked on her door since she placed a “Not For Sale” sign in her 23rd Street window three months ago. Before then, Ms. Morciglio said, she received offers for her threestory frame house “almost every day.” When she purchased the house 22 years ago for $10,000, she had a clear view of New York Harbor from her front stoop.Two years ago, an eight-story subsidized housing development went up on Fourth Avenue, obstructing her sight line. Still, by this spring, she was regularly receiving offers for the house that topped $800,000.

Meanwhile, some residents who initially put up the signs to deter unwanted inquiries from realtors say their effort has backfired. “I had too many people come up to the door, ringing my bell, and asking me why I don’t want to sell,” Anne Perrotta, 83, said. She ordered her husband, Anthony, 84, a retired sanitation worker, to take the sign down.”Anyhow, I had to wash my window,”she said.

Ms. Perrotta has lived in Greenwood Heights for nearly six decades, and she vows to remain in the neighborhood for perpetuity. “When I retire, I’m going to retire across the street,” she said, a reference to the location of her home, at the edge of sprawling Green-Wood Cemetery, the final resting place for a pantheon of famous New Yorkers, including Leonard Bernstein, Horace Greeley, and William “Boss” Tweed.

In their fight against rapid development, residents invoke the neighborhood’s storied history as the site of the Continental Army’s valiant but ill-fated stand against better-equipped British redcoats during the Battle of Brooklyn in August 1776. And earlier this year, hundreds of community members rallied against a planned 70-foot condominium tower on Seventh Avenue that would have blocked the view from Battle Hill toward the Statue of Liberty. Minerva, the Roman goddess of wisdom, has stood atop Battle Hill for 85 years, and she gestures with her left hand toward Lady Liberty three and a half miles away. After the protest, a sales agent for the Seventh Avenue project, Elan Padeh, said the proposed condo tower might be scaled down. Mr. Padeh, the chief executive of the Developers Group, a DUMBO-based firm, did not return repeated requests for comment.

Elsewhere in the borough, Brooklynites who are battling against development look toward Greenwood Heights’ “Not For Sale” movement for inspiration. Signs have sprouted up in Bensonhurst and Windsor Terrace. And Mr. Brashear said he gave away several signs to Develop Don’t Destroy, a group formed to oppose real estate mogul Bruce Ratner’s plans for a high-rise urban hub in the downtown Atlantic Yards area. “It’s the right way to tell people to leave you alone,” the group’s spokesman, Daniel Goldstein, said. But for members of Mr. Goldstein’s group, simply refusing to sell their homes might not be sufficient. City and state officials have endorsed a plan that would use the power of eminent domain to evict the roughly 20 residents of the Atlantic Yards area who have spurned Mr. Ratner’s bids.