Well I so hear you because ds had some 'friends' like that. Role playing worked well for us, even though ds hated it. I would pretend to be the mean kid, and say things I knew the mean kid was saying. Ds is so nice and mild mannered that he wouldn't even be able to tell the kid when he came to the front door that he didn't want to go out and play with him. So I made it a game and pretended to be the mean kid, rang the doorbell and had ds 'practice' telling the mean kid that he was not going to be coming out.
Whatever you do though, my advice is to make it pretty generic. What I mean by this is, say when the mean kid came to the door, I wouldn't have ds say (even though it was true and I wanted to) "You are not a good friend and I don't want to come out and play with you". Instead I just had him practice, "I can't come out right now, we're leaving soon". It was not a lie because we were always going somewhere. Going into specifics, even if they are true, just to me doesn't seem like a good idea.
Oh and my heart goes out to you, because I have been in your shoes and it is gut wrenching and nerve wracking. Eventually your ds will 'discover' these are mean kids and he just won't want to play with them. But as your child IS a good friend to their friends, the concept of 'mean' friends cannot be taught. Eventually the light will dawn, and be prepared for that too, because that is Really hard. I had my speech all ready for when this came last summer. It was a hard few days but it does get better.
Personally I'd try to start lining up nice playdates for your ds now before summer starts. The summer is very long and lonely for a nice kid with mean neighbor kid 'friends'.