Very difficult subject...
Tackle the Problem from the Inside
It's the most effective, and the point you have the most impact on. It sounds possible zen and overreaching, but the child need inner-strength, and, above all, inner-peace.
It's going to be very difficult, but someone would need to help the child to be more passive about the assaults and not show an emotional reaction towards them. Obviously difficult in the general case, and even harder here, I know.
And at the same time, the child will need to learn to resist them as non-aggressively as possible. It doesn't mean fighting back, it doesn't mean responding in kind, it means making it less easy.
It's very hard and sad to say, but it's about making it less fun and more difficult for the attackers.
Tackle the Externals Factors
I'd also think that if these things are routine enough and happen within your (or anyone's) sight, it's your responsibility to walk up to these kids and try to talk to them.
Use the Golden Rule
As mentioned by w00t in his answer, they do it for the sense of belonging to a group, so it's a difficult dynamic to break. But it's quite likely some of these kids have someone in their close or remote family circles that have disabilities and that they wouldn't them to be picked on. They also wouldn't want any of that to happen to these relatives or themselves, disabilities or not.
If you can get them to imagine what it would be like to live in the other child's shoes, if only for one day, it will it home. It won't be enough to beat group-effect in most cases, sadly, but it's a step in the right direction. Change one at a time.
Go to the Source
However, you can't handle other people's kids, and it means there are other people who need to get involved. Their parents, their teachers, or other educators or authoritative figures they look up to.
I'd even recommend to go as far as to talk to educators and collaborate with them to build sessions to raise awareness, if that's what it takes. You will turn some around, and these will then be helpful forces in preventing the more die-hard bullies from keeping at it. Organize a presentation at school on the topic. Invite kids to a home or bring someone with experience to talk about the issue - and make sure the kids actually talk to this person, not the other way around, as they'll be force to create a bond.
Maybe screen videos about the impacts of bullying on the life of people with these problems.
These usually don't let people flat out cold. They may pretend to laugh about it and snicker while in groups, but they'll definitely get something out of it.
Actions have Consequences
It's also probably a good thing to make them understand that their actions can have consequences. Not just for the kid, but for themselves, as the child and the parents could press charges, and while it's fair to assume the bullies are juvenile they will still have some degree of obvious discomfort in that process.