Has this ever happen to you guys where a guy confess his love to you, but you have to break his heart because you aren't interest in him? 

it happen to me where I was friends with someone's friend and he keeps on calling me up and telling me that he misses my company, wishing that he was there for me, always telling me how kind and sweet I am. I fear that he trying to confess his love to me and I am trying my best to keep a distance away from him and also telling him to go off with someone else 

There's no good way to do it, but in my opinion, less it best. Don't try to sugar-coat it or soften it, just tell him kindly but frankly so you can both get on with your lives.
Also, I would advise against the whole "let's be friends" bit.

i have try to do that, but somehow someone is trying force him to date just because i had a rough break up 

Just come right out and say that while you like the attention, you're not attracted to him and not interested in dating.  Being frank is best, I think.

that is true. now i dont date since after my break up, i am free to be single and staying focus on my school work 

It happened to me before. He was such a nice guy though. I felt HORRIBLE! I still say "Hi" once in a while and he does to me too but there's still this awkward vibe (in which I'm trying to get rid of)!

i know what you mean. he sometimes he says hi to me when he calls me up and telling me that he misses my company and it was kinda ackward to have that vibe 

Yes, and I don't have feelings for that guy. At first I felt bad for saying that I don't like him but I know in my heart that it is the right decision that I had made.

The best way to let a guy down is to be kind, but just like RozeMaiden said, you HAVE to be frank. Guys are very black and white. If you say something sugar-coated and soft, they might think they still have a chance. You need to tell them the truth, and make sure they understand it. But don't be rude, and don't be mean unless it's warranted.

I've had to do this. It was...hm, maybe 2 years ago now? I thought I liked him, but realized I was just flattered by the attention and wanted to have a guy that liked me. I came to this realization pretty suddenly, and the moment I did, I told him exactly what my real feelings were. It went kinda rough. I'm not a mean person, but I can be pretty harsh. And I had to be harsh, because he just wouldn't accept the fact that we weren't in love. In the end I had to be brutal, but I didn't let myself feel bad - he needed to understand that I didn't love him! I actually had to go as far as to say "Stop being such a sap!"
We didn't talk for a LONG time after that. But a few months ago, he contacted me and told me that he'd changed, and that he was sorry for the way he treated me, and he was thankful that I tried to help him but didn't let him use me, and broke it off before things got serious. He has a girlfriend and they're very happy, and we've become good friends again.
So don't worry. Letting a guy down, no matter how horrible it goes...it's not the end of his world. He ought to be thanking you, in the end. My friend is proof!

To some guys, telling means nothing.  To me, I wouldn't care about my reputation and start acting the opposite of whatever I normally do when he is around.  

If he is a nice person, he would understand that he would need to wait until you accept him in your relationship.