Saartje, whatever you do, do not send an email to him saying 'no'. Do not tell him you are not interested in him anymore with an email. That's a slap in his face. Meet him and tell him the truth, if you are sure that you don't want him then there is no point in giving him hope that you might change your mind someday. Yes, it will hurt, but the faster he finds out he has no chance, the faster he can move on and find someone else. I would think that that is better than giving him false hope.
But hey, I'm no relationship coach, so 'my method' could be the worst.