Love is not by force. If you think someone is not compatible with the person, you can simply let them know.
It is better you let them go, than to keep lying about your feelings to them.
The best way to do that is to give reasons why you guys won't work.
Maybe take him/her out and tell them how you feel about the relationship.
Let them know that you didn't come to the decision so easy, that you had to think things through and it wasn't easy.