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All I can say, is that you just have to tell him. And if he doesn't take it well... then, there is nothing you can do about it.

Like Shinny said, wait for Val or Chibi to post.
They give really good advice.

And I'm the other one they're referring to. xD Let's see....

Well, you're in a complicated situation, and I don't think letting Ace down gently is going to work. You said he knows that you have a boyfriend, but that he's not really respecting the boundaries that you now have. Well, it sounds like Ace either a) doesn't realize that you are serious about Ben, or b) thinks that because Ben is far away, you're not really 'with' him. I think you need to sit down and have a long talk with Ace. Kindly but firmly explain to him that you do like[love] Ben, that you are not interested in being with Ace, and that you wish he would stop [doing whatever he's doing] because it makes you uncomfortable.

One thing you really need to do is to put emphasis on the fact that you're not interested in him. Don't make it seem like you're using Ben as an excuse for not being with Ace. Tell Ace straight out that you wouldn't be interested even if you and Ben suddenly broke up. Don't give him false hope (like saying, if we broke up some day, maybe then...). But if he asked (what if you guys break up?) just say something like, "I would have to re-evaluate my feelings at that time. How can I know now what I will feel then?" Be concrete when it comes to the now and vague when it comes to the future.

There is no way to let him down so that you two will automatically still be friends. It doesn't work that way, sadly. He may pull away from you a while as he nurses his broken heart, but eventually, if you guys were real frends, then he will come back to you. It just takes time. Please don't say 'can we still be friends?' because that is very painful and usually has the opposite reaction intended. Basically, be sympathetic but firm, and let him know that if or when he's ready to be friends again, you'll be there.

I second Val on the next situation - this sounds like Newly-Wed syndrome, wherein newly wed couples ignore their friends for weeks after they've been married. New relationships are exciting, and all of them go through a period when you want to spend as much time with that person as possible. He's not doing it on purpose; he's probably not thinking 'I am pushing my friends away' as much as 'I am spending time with my new girlfriend'. You have to look at if from that perspective to understand [case in point: when Ben comes to visit, aren't you more focused on him and less focused on everyone else?] You can either wait for this phase to pass, or make specific time to hang out with him.