What can I say in order to avoid being manipulated again?

Over the course of 6 years my sister-in-law has been the biggest thorn in my life. I have always been very courteos,considerate and helpful to her. I have always been there when she needed a sitter,advice,etc.She is never there for me and is a big manipulator. Just recently I decided I had enough and I have distanced myself from her. I do not call her and when she is around I try not to engage in any conversation. I have been doing this for 3 wks and I think she has gotten the mesage. However, in the past when I have done this after a while she just calls and is really nice and pretends nothing ever happened and I always fall into that trap. This time, when she calls me and acts like everything is ok, whatcan I say to her to get the message across that she broke the camels back one 2 many times without engaging in too much of a conversation??? Or should I just keep the talk really short? Thanks for any advice--10 points for best answer!