What to do when a former supervisor keeps contacting my current supervisor as a way to pressure me to publish my Master's thesis?
One thing you should understand is that in this crazy world, she may benefit from having your thesis published more than you, especially if she is currently under considerations for reappointment or promotion, so don't be too hard on her: rather blame the entire screwed up evaluation system in academia that has made much stronger people panic and do ridiculous things.

With all that said, the usual advice applies: be firm but polite and show some good will. Think of when you can realistically finish the job (it is in your own best interests to finish it eventually regardless of anything), and tell the plan to your current adviser. That should settle your scores with him and, most likely, he'll no longer bother you with that anymore. If you want, you can tell the target date to your former adviser as well.

Don't forget to give yourself some leeway, so that you can keep your word no matter what. Remember that promising less than one expects from you won't hurt your reputation, but failing to keep your promise will.