I think that being nice to him was a really good thing to do first, unfortunately, he apparently didn't exactly get the message. I would suggest telling him again, but still in a nice gentle way. If he, again, fails to get the message, be more assertive.
Remember, assertive and aggressive are two different things. Assertive is where you stand up for yourself and your beliefs, but without being mean and cruel. Aggressive is actually being mean, actin like a bully and physically (potentially) harming another person. So remember, assertive not aggressive.
Also, I would suggest not ignoring him or anything along the lines of treating him different that what you usually would as a friend. You probably don't want to lose this guy as a friend, and just ignoring him by pushing him away will not solve anything and could possibly jeopardize the friendship you two share.
Mainly though, it comes down to you and how you're feeling about this. If you're feeling annoyed by this, you tell him. If you're feeling uncomfortable about this, then tell him. It's not all about you giving in so he can be happy, it goes both ways, 50-50 (well if you wanted to, I guess you could say yes just to see how it goes and what not, but that's really up to you). You've got to tell him that you don't see him in a romantic way and you only see him as a friend and it won't go past that.
If it really gets to the point where you are stressed out because of this, instead of lashing out with harsh words and what not, just tell him "Stop". Not a timid "Please, stop", because that probably wouldn't work if it elevated to that point. Just a plain "stop". Hopefully it shouldn't get to that point, but ya never know.