Cull them. If you don't it will spread to the rest of your birds. Even if you treat they still carry it and will spread it to your other birds. It's airborne and spread on fomites. I had it once and it made it from my workshop where I brood my chicks all the way from one half of my property to the other half and infected my marans flock before I caught it and culled everyone. If you don't cull you're going to have to consider all your birds carriers and run a closed flock, which means no selling birds, chicks, or even eggs (since it is actually spread right through the egg to the chick and chicks hatch infected with it), no taking birds to shows where you will infect other people's birds, etc.
You'll also want to keep a close eye on your uninfected birds now. Mycoplasma has up to a two week incubation time so newly infected birds won't necessarily show symptoms right away.
What makes you think it's mycoplasma? What symptoms do they have? Have you culled one and sent it off for necropsy? You might want to confirm it's something this serious before culling, although if they're showing all the classic signs I'd personally cull now just to be safe and then send one off for necropsy to confirm before it has a chance to start spreading, especially since they're meat birds that you intended to kill at some point anyway and not a rare heritage breed or anything like that.
