I think you guys are being way too nice on the subject.
The challenges on this site SUCK. Sorry for the crudeness, but better words fail me. Everything is a popularity contest, and the more friends you have that vote unfairly, the more chances you have of winning. People on this site are initiating challenges with cars that aren't theirs, cars they no longer own, and as in one case I saw today: A toy car was beating a real car.
C'mon, now; is this for real? I won't turn down a challenge from anyone, because I think it is humorous. Unless you plan on voting against the better car, and leaving a comment on the challenge board so the guy know you voted for him and he can return the favor; don't even get in on the challenges.
Every now and then, I'll pop into the challenge board and leave some comments about how ridiculous everyone is, and then I'll disappear for a few weeks. lol