What to do when a former supervisor keeps contacting my current supervisor as a way to pressure me to publish my Master's thesis? I'm all for publishing my own work of course. Since I have coursework and new projects during the semester I have temporarily put aside my manuscript and I have communicated with the former (Master's) supervisor about this. But now she kept contacting my current PhD supervisor and he in turn has written to me a few times regarding this matter. 

I feel my former supervisor's behaviour is quite innappropriate and this is the third time she does this. There have been two very similar instances previously--her asking my PhD supervisor whether I could depart later than my planned date to start my PhD program; then about two weeks before the final thesis submission, she contacted my PhD supervisor while I hesitated to let her use my thesis for grant application--both times behind my back with me only learning what she did from my PhD supervisor. I called her the second time she did this asking her very politely not to do such a thing again and hoping to remain on good terms. She agreed. But now this happens again........

What can I do to stop her from doing this? While not having my relationship with the current supervisor damaged but also I guess not having to be coerced into doing what she wants? 

Any thoughts or suggestions would be appreciated!
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.