Gotta agree with everyone else here that you might not want to cut them off just yet. Sometimes attraction doesn't emerge right away. A lot of relationships that start in the "real world" are like this. Two people meet and don't particularly feel attracted, but as they get to know each other, something blossoms. I think the Internet has spoiled us to an extent and made us believe that all attraction has to be instantaneous.

That said, if you know they're just not and never will be your type, the Internet makes it pretty easy to let people down nicely. You can end the actual date by saying something noncommittal like, "Thanks for the coffee. It was nice to meet you." If the guy follows up by asking you out again, that's the time to respond with something like, "Thanks for the invitation. I had a nice time with you, but unfortunately, I didn't feel a connection/spark/sense of chemistry. But I wish you the best of luck in finding the right person!" Or, if you think you might want to be friends, you could say, "I had fun, but I think we'd be better as friends. What do you think?" (You should only say this if you really want to build a friendship, though, not just to be nice!)

Most men will take no for an answer, but if one or two persistent ones pester you for explanations or second chances, you're not obligated to answer them. You can ignore them if you want.