You can mark the request as "spam" and that person won't be able to contact you again. (I just tried it out on a random person who sent me a request to make sure.)
I have zero problems hitting "delete". One of my DH's crazy aunts by marriage tried to friend us. He and I both hit "delete". His sisters and mom didn't. Boy, did they live to regret it. It's always the tangentially related people who cause FB drama IME.
If someone friended me just to sell me something I'd unfriend them as fast as I could!
Mommy to my wonderful, HEALTHY twin girls
6/08 - Preemies no more!