Sebelia, 36, responded on myspace.com to a report that he broke the reality show's rules by seeking outside sewing help, The New York Post reported Tuesday.
On the Web site, he warned fans to "be careful of what you read and also of what is presented in the press."
"Those people take any information they get (usually unfounded rumors) and just print them in order to bolster their own readership with no regard to the person they might be slandering," Sebelia wrote.
A show spokesman refused to divulge the outcome of an internal investigation into the claim, The Post said.
"We took the accusations very seriously, and we did a very thorough investigation," he said.
"Project Runway" will air its special two-part finale on October 11 and 18.