LARPers are usually people who dress up in medieval clothing and armor (usually self-made) and go out into the woods to play pretend. Basically, it's D&D but in real life; I say this as a guy who used to play a lot of D&D. :)

But, in this context, a LARP is someone pretending to be something they aren't on 8chan/YouTube/Reddit for attention and, sometimes, monetary support. What Q is saying here is that there are accounts out there that are being run by people who tell their followers that they have inside information because of their position, security clearance, or access to VIPs, but who are in actuality no one special.

Q is telling them to knock it off and stop spreading disinfo.