If you're going to allow sit ins, especially those that are popular with the local crowd, then it's best to just have fun with it, even if he sucks. IMHO, flipping him the bird and otherwise being rude to him may actually cost you some fans.
+1
It's always very awkward when you're put in that position, but sometimes you just have to grin and bear it. There are times I've had sit-ins that I literally wanted to bust out laughing right there on stage, but I always just keep a straight face and get through the number. While certain approaches to sitting in can be classless, disrespecting the person who is sitting in can be classless as well.
Case in point, I remember playing an Italian club years ago. The band I was in had a steady gig there and the club manager really liked us. He asked if some friend or family member could sit in, so we obliged. Our singer and rythmn player let him use his guitar. I don't think the guy ever really played in a band before, because it was like a kid going to rock camp for him. I still can't forget when we played the opening chords to Gloria and he proceeded to rock out like he was at the Civic Arena, striking poses as his wife took pictures. He wasn't a bad player, either, but he played these wanky, self-indulguent penatonic leads that just went on and on. The whole thing was funny. I wanted to laugh, but I just rolled with it and had fun. We took care of the club manager's guest and had a great relationship with that venue for the rest of the band's tenure.
And I hope that "I Hate My Life" is one of your strongest songs!
I like Theory of a Deadman, but that tune isn't exactly rocket science.