I hope you don't mind if I assist on this one? @Bit
This is a tough situation to be in man. You need to find a medium between passive and aggressive. Be firm. Keep a calm composure on the outside, but let people know that you're not playing. It also helps to be consistent. If people don't know how you're going to react, or they think there's a chance that you'll let them walk over you, they'll try to seize the moment.
So in review, be firm and be confident. Know that nobody can dictate your happiness but you.