Hello Tango Mango 123 & welcome.
When you say that this person 'likes' you, I assume you mean in as romantic manner as opposed to a friendly manner?
And when you say that you 'don't like' them, I presume you mean that you do like him or her, but only as a friend and not in a romantic way?
I think that you just have to be honest and say that you value the friendship but don't want a romantic attachment ~ and actually say that you don't want this to have a bad effect on the friendship.
However, it might be hard dealing with this for both of you.
Judging from other posts, this sort of thing happens quite a bit.