General question: How to say "No"?

[taelifoe.livejournal.com]

Hey all! First time posting here, so please forgive me if I come across as a bit awkward. I have been reading through a lot of the stuff in the A_B community for a little while now, and being someone who has started taking commissions, it occurred to me that sometimes I may have to turn away potential clients who have a troublesome reputation.

My question to the community is this: If I'm approached by an interested client who I KNOW is going to cause problems for me, how can I bow out gracefully? Obviously, I don't want to be in bad form and say something like, "Well thanks but no thanks, I've been told you're a complete (insert rude name here) and I couldn't stomach doing business with you."

So......advice, anyone? Thoughts? Thanks in advance!