Obviously in our line we have to be prepared for all sorts of things. I don't discriminate by shape or size, and obviously we often don't know until the door is opened anyway.
Attitude and respect are the biggies and it's becomes easier to spot this over the phone, and by email or text(although less so) before the appointment happens. If I sense something wrong, or that the guy really isn't suited to me then I'll decline the booking. I tend not to fib, I just say "Sorry not possible" or "Let's leave it". There are however some guys who come across as having an attitude problem, or not clicking on the phone who in person turn out to be great. It's just spotting them that's the problem.
I won't accept any booking which makes me feel uncomfortable, doesn't give me enough time to go through my usual security, or an outcall without all the details I require. I'd rather turn away the booking and the potential fee, stay at home safe and sound than run any more risk than I have to.