'NO.' The most useful word anyone will ever learn. When you learn a foreign language learn NO first and leave yes for the advanced course. Use it liberally, shake it on your food like salt, use it to punctuate sentences. If you get tired of sounding like a toddler, mix it up with "Hell no," "Hells to the no," "Are you fucking kidding me" and "Clearly you have me confused with someone who gives a shit."
"Fuck You" should always be close at hand for stubborn situations. This pithy phrase enforces boundaries better than any Iron Curtain. Wildly underutilized by the competitive-mothering crowd, it will really drive the point home. Try it at your next PTA meeting and watch the fun.
When the situation really gets out of hand, do as I do and slide into a depressive torpor for three weeks to four months. This always clears my plate in a hurry. When it's over, those who love you will be glad to have you back, and those who don't will never bother you again.