The Matter of respect
To say "NO" those people who use you it is a matter of respect. It is not a secret that the first on the way of respect is to respect your own self. In case you have self-respect other people will also respect you. Those people who let other people use them are people of no self-respect and so they are people who will never be respected.
There is no need in respecting people who can't respect their own selves. See, the attitude of other people towards us depends on our own attitude towards our own selves. People used to say that one has to treat other people the way he or she wants other people treated him. Though, this idea is wrong when talking about those people who fail to say "NO".
Respect yourself and realize that you time, your life and work you do is of great importance and of great value. Relies those simple things and make other people relies those and things will get clearer at once. Try to view this situation like you are an onlooker. See a guy who always does job for other people and than sits nights long doing his own job?
It is you, it is you are a person who spend its nights for doing own job because a day time is occupied by dealing with the problems of other people. You are loser and people use you like they use their computers or something. Stop being a loser and make people respect you, make them respect you and have your time for you.