I read a question in a forum today and it resonated with me. The poster's issue was that he (or she? not sure) has a coworker that always comes to him for technical support. He has tried several avenues to get her to follow the proper protocol - contacting their company's service desk and go from there - but she continues to come to him. He doesn't mind helping her, but everything to her is an emergency and his responding to her troubles has placed stress on his own workflow.
It resonates with me because I've never been able to say no to people either. It used to make me feel helpful and important. Until I realized that people were (usually) only nice to me when they needed something.
The best response included this quote:
This is a person who likes to take the path of least resistance.
Don't be the path of least resistance.