<human>: I've been friends with this guy David for about a year now. We actually work together, on a very small and tight knit team, so we are also close friends. It is important to note that he's incredibly socially awkward, and it took a long time for us to become friends because of this. He isn't very in touch with himself emotionally. Recently, he's been dropping hints that make it seem like he likes me as more than a friend, but I've tried my best to shut those down. A few days ago, he asked me to go do something together in a way that seemed like he was asking me out. So I called him out on it directly – I asked him if he was asking me out. He sent me a long message about that he "guessed he was" but that he didn't want things to get awkward or anything between us. 
Unfortunately, although he is a great guy, I don't want to be with him – I actually like one of our other friends, Colton (making the situation even more convoluted!). I tried to let him down gently, and reassured him that I still wanted to be friends. However, since then, he keeps bugging me about it. I've tried my best to act normally and interact with him as I usually do, but in response to that he has begun flirting with me heavily. He wants to get together and talk, just the two of us, about the situation. I really just want him to let it go, so that we can return to being friends, but I know that it can't be easy for him to just ignore those emotions. However, I would also feel guilty dating Colton, because I know it would hurt David. 
One other relevant detail is that David will actually be changing jobs in the middle of fall. He doesn't know where yet and he might end up in the same city, or he might end up far away. Thus my question – with the goal of actually preserving the friendship, even after he leaves the company, how do I handle this situation? What is the best way to approach the issue without damaging our friendship, but also making it clear that I'm not interested?

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: My close friend and coworker likes me but I am not interested in him. How do I stay friends but turn him down?