You fundamentally can't force people not to speak. Or, rather you can, but you very rapidly move toward ethical and legal problems. What you can do is take away some of the energy that encourages them to speak. By putting distance between yourself and the gossip, you reduce the energy and freneticity of the topic, and people move on to something else. When people gossip about you, it's like being the subject of a news story. The less you give back to the story, the less interesting it is, and the quicker those who thrive on being the center of attention will turn their attentions elsewhere.
While I very much like John's answer for some if its direct communication, I have some suggestions to follow along-side--not in replacement of--John's suggestions. The first two steps are fairly passive, and not likely to escalate existing tensions; they're designed to retain relationships by reducing amplifying feedback. The third step involves asserting positive, reasonable authority over your own little corner of Facebook, and the final step involves out-right removal of them from your online life, to the extent that that's possible:
The first step is to lock down your Facebook profile. To the extent you have any passive control over what people say about you on Facebook, that control is available via your privacy settings. Disallow people from tagging you. Disallow people from writing on your wall. This reduces an agitator's ability to push their message at you in a public way that's likely to get amplified by comments and such.
Second, unfollow agitators. Facebook doesn't show everybody everything happening about all of their friends. It tries to maximize interaction by looking for people who interact a lot when presented with each others' posts and comments. You can hide (most of) agitators' posts and activity by unfollowing them. This is distinct from unfriending; you can still see each others' content, but you won't see theirs unless they post somewhere else you were already looking, or unless you visit their profile. By not being presented with their messages, you're less likely to interact with their material, and so Facebook is less likely to show them yours.
Third, you can lay down rules for your Facebook wall. It's your private space, and it's under your control. If they engage in these behaviors on your wall, you can ask them politely not to. I wouldn't expect immediate compliance, but I wouldn't respond to any immediate retort of theirs; that way only lies escalation, which is the opposite of what we want. If they do it again later, remind them. If they refuse to follow your rules, and you've given them a couple explicit opportunities and some time to let those opportunities to sink in, it's time for the next step. (It's important not to rush, as rapid progression from "whatever" to "goodbye" gives the impression of being irrational and unreasonable to those aware of the event. No matter how unfair those assessments are, they still make social interactions awkward.)
Fourth is simple unfriending. Given how promiscuous we tend to be about friending people on Facebook, unfriending tells them they're not even worth your passive, occasional attention, and you really don't care about them online. It's going to sting them, but that's not the reason we do it. We do it because it prevents them from being able to be jerks on your Wall, after they overtly insisted on being jerks, or were insistently obtuse and un-self-aware about their being jerks.
Remember, step four is going to sting them. Expect some fallout from that. If you can, try to go a week before taking any further action; hopefully any upheaval in your social circles will have cooled some in the interim.
If they still insist on being jerks on Facebook, there's not much you can do to prevent it. But you can limit how much you let it stress you out: You can block them. Blocking them prevents them from being able to see your posts, and prevents you from seeing theirs, including their comments.
Hopefully, you don't have to get all the way down here. Time and ignoring gossip can go a long way to making yourself uninteresting to them, and finding ways to hide their activities can help reduce your stress levels.
Again:
Step 1, make it harder for people to draw you and your other contacts into their gossip.
Step 2, pay no attention to the gossipers, and Facebook will tend to naturally draw them away from you.
Step 3, if the gossipers are deliberately bringing their trash to you, you can positively assert authority over your personal space, but you need to be perceived as calm and reasonable about it to reduce the blowback.
Step 4, if the gossipers truly won't take "leave it alone" for an answer, all you can do is block them.
