A judge ruled Monday in favor of “Harry Potter” author J.K. Rowling in her copyright infringement lawsuit against a fan and Web site operator who was set to publish a “Potter” encyclopedia.

U.S. District Judge Robert Patterson said Ms. Rowling had proven that Steven Vander Ark’s “Harry Potter Lexicon” would cause her irreparable harm as a writer. He permanently blocked publication of the reference guide and awarded Ms. Rowling and Warner Bros. Entertainment Inc. $6,750 in statutory damages.

Ms. Rowling and Warner Bros., the maker of the “Harry Potter” films and owner of intellectual property rights to the “Potter” books and movies, sued Michigan-based RDR Books last year to stop publication of material from the Harry Potter Lexicon Web site. Mr. Vander Ark, a former school librarian, runs the site, which is a guide to the seven “Potter” books and includes detailed descriptions of characters, creatures, spells, and potions.

The small publisher was not contesting that the lexicon infringes upon Ms. Rowling’s copyright but argued that it was a fair use allowable by law for reference books. In his ruling, Judge Patterson noted that reference materials are generally useful to the public but that in this case, Mr. Vander Ark went too far.

“While the lexicon, in its current state, is not a fair use of the ‘Harry Potter’ works, reference works that share the lexicon’s purpose of aiding readers of literature generally should be encouraged rather than stifled,” he said.

He added that he ruled in Ms. Rowling’s favor because the “‘Lexicon’ appropriates too much of Rowling’s creative work for its purposes as a reference guide.”

Anthony Falzone, who argued the case for RDR Books, said he had not yet seen the ruling and could not immediately comment. RDR publisher Roger Rapoport did not immediately return a telephone message for comment. A spokeswoman for Ms. Rowling and Warner Bros. did not immediately return a telephone message for comment.

Though Ms. Rowling had once praised the Web site, she testified earlier this year that the lexicon was nothing more than a rearrangement of her material.

She said she was so distressed at the prospect that it would be published that she had stopped work on a new novel. “It’s really decimated my creative work over the last month,” she said during the trial in April.

If the lexicon is published, she went on, “I firmly believe that carte blanche will be given to anyone who wants to make a quick bit of money, to divert some ‘Harry Potter’ profits into their own pockets.”

Mr. Vander Ark, a devoted fan of Ms. Rowling, began work on his Web site in 1999 and launched it in 2000.

The seven “Potter” books, which ended last year with the final book in the series, “Harry Potter and the Deathly Hallows,” have been published in 64 languages, sold more than 400 million copies, and produced a film franchise that has pulled in $4.5 billion at the worldwide box office.