Do you enjoy frustrating such a friend? I may have some tips, but whether they are relevant depends on how you answer that question.

I'm assuming that you aren't sexually interested in them? Be very, very clear about how you feel about them and that you aren't interested in a sexual relationship. Not "I don't want a relationship right now" but something like "I value you a lot as a friend but I don't want our relationship to be sexual". Establish boundaries about what you're comfortable with in your relationship with them, and stick to those. Talk to the friend, ask how they feel about you wanting to stay friends, and what they need/what you can do for them in order to maintain the friendship. Understand that even if they agree to only be friends, they probably can't make their feelings go away overnight (if only feelings worked like that!). I think honest communication (in both directions!) can help a lot in this sort of situation.

If the person decides that they can't stay friends with you if you don't reciprocate their feelings, or if they need some distance because of it, that's absolutely not your fault. Only 50% of the friendship is on your end; it's their choice whether they maintain their end and accept what you have (and don't have) to offer.