Your best tactic in such a case is to show where the statement is incorrect. Attacking the poster though, calling them a liar as an example, does not advance the discussion and will result in sanctions.
There is no problem showing someone where they are wrong.
There is no doubt that this is a difficult task on the part of both the poster and the moderator. If, in response to such a post you said the message displayed ignorance of the facts or was bigotted in nature, that would be within the guidelines. If, on the other hand, you said the poster was a fool or was lying, that would be outside.
Aslan is not a Tame Lion