[quote=Inuyasha]
I feel that if I outright reject them I will look like an asshole to the other players.
[/quote]

You're not in control of that part.

You're in control of whether or not that player joins your RP, and, to an extent, whether you decide to put up with them/help them out or not to have to. That's the decision you have to make.

Second Kestrel's point. Be specific about what that player is doing (or not doing) that makes them unsuitable. At least that way they have something to look at and try to work on if they want to.