To this good [answer](http://moderators.stackexchange.com/a/756/83) I would add: consider creating a page on your site where you (dispassionately) lay out the facts.  Don't name the other site (why give them publicity?) and don't get into "they said, we said" -- just say that you take copyright seriously, violations will be dealt with, here's a link to report them, etc.  Do that for each category of false rumors/claims.  Then, when accusations are made, you can just say "that's incorrect" and provide the link, instead of engaging in the discussion each time.

If all of the claims are about policy issues, then instead of a separate page consider making this part of your TOS, which I assume you publish on your site.

This [related post](http://communitybuilding.stackexchange.com/a/676/83) talks about a user within a community rather than external to it, but many of the responses are the same.