Just tell them straightforwardly.
I know some people who aren't straightforward and they end up hurting others. 
There's one person I know who's very friendly and very likable. Unfortunately, she's too friendly in a manner that would make guys think she's interested in dating. She will go out with them to parties, dinners, clubs, etc... then she moves on to the next person like she's just busy with friends. She doesn't think that she's hurting anyone, but a lot of guys have gotten upset when they find out they wasted their time trying to date her when she was never interested in the first place. :roll:
