I dont think this is a worthy reason to recieve a ban because it would be lame if the host says a mode and the blue player sets a different one, people joined to play a mode that would be changed afterwards.
I believe they are in their right to leave and of course nobody's going to take a screenshot of what mode host says I suppose that should be a call for the host himself, but considering it barely ruins the game since it just started I dont think it's worth the effort.
I'd suggest to solve these sort of cases without getting any mod involved, fast remake & done.
[14:17:09] [System Message] Raigor[EST] can access this room from now on!