What is the point in wasting your time on people who presume upon you?

What is the point in wasting your time on people who presume upon you? I always fall into that trap. I try to help those I consider being friends. And they take advantage on me and on my generosity. But I merely fall into wasting my time, wasting my patience and goodwill. Attributes and personal features that I could easily dedicate to those who really love me and who take care of me and do not use me as if i was an object. People that call me just because they need a special information and then end up without even asking me how I feel. You are lame, malicious and self-centred.