I know you want to do the right thing.. but you sincerely can't let people like that ruin your life. You want to look out for others.. but who will look out for you?
I'm not saying that you should be entirely selfish and screw over everyone else, but.. you can only take so much and give so much before you need to look out for yourself. As they say: "Fool me once, shame on you. Fool my twice, shame on me"