When they take your time and resources without permission repeatedly, it's a problem. You can write them a letter or an email explaining your feelings. Detail the times when they made you feel like a slave. Suggest ways to improve the friendship, like sharing tasks equally or respecting each other's free time.
A sign could be that they always make you do things for them without any reciprocation. What you can do is to distance yourself a bit. Don't be so available all the time.
If they constantly boss you around and don't listen to your opinions, it's a sign of this 'forced slave' situation. Try to find mutual friends to mediate. Let them know how the situation is making you feel. For instance, you can say 'I'm tired of always doing what you say, I'm not your slave.' This might shock them into realizing their behavior.