Generally you have to just deal with it as GM. Of course if they get out of hand you can tell them to leave or tone it down, but I don't know if this person goes that far or not. If they don't stop or they keep posting even with them warned and/or dropped from the RP, then the mods are there to do something. The only way is not to accept them or if they argue anyway is to go to the mods. Remember you are the boss, the master and its your story, not theirs. 

Well that is just me, maybe someone else will have something better to say. I am sure the mods will.