Well something similar has happened to me before, I knew clearly that it was edited (but he edited it fast so it never said edited). I'll retract my 'Liar' comments but there is no proof to say he did that. When it happened to me I just ignored it, knowing all to well that I wouldn't win if I argued because there was no evidence to support it. It's only one comment on a forum, not the end of the world you know :/.
Seriously I wouldn't rage at the screen just because someone called me a liar, you don't have to prove yourself to anyone, as long as you know the truth and believe in it as well that's enough.