No, I don't want to be your friend

I am a member of several of the social networking sites. On one of them, I have made some cool connections to the past. Except one that should stay there.

There is a person in my past who hurt me a lot, probably without meaning to because of their own issues, but there it is. I'd like to say I don't hold a grudge, but I do. Well, this person keeps sending me friend requests on said social networking site. I do not want anything to do with this person and keep denying the requests. Should I tell them why?