i've been in such situation..
So just tell hir so. If you're in generous mood, give hir a week (or 2?) to switch. But it'll probably be a waste.
btw, consult the team about it. If they also say s/he's more trouble than helping, then bye-bye. You cannot afford "negative" performance - someone reducing others performance in this or that way. The team might be better without hir - more work to do but at least no fake stuff..