Keep your cool and keep practicing being assertive. Probably one of two things will happen. Either they will begin to enjoy this new kind of communication and the new respect you show for yourself and for them or they will not be able to stand it and will move away to find new friends. If the latter happens it will may well be because they have found out they cannot manipulate you anymore and will find someone else to use. Wish them well.
People often tend to be nonassertive because they are confused about what humility is.
We want to be humble at all costs and sometimes what we think of as being humble can exact a high price of unhappiness and lack of fulfillment on our part. On the other hand, some people are very proud of their humility. We also believe we must avoid being thought of as selfish and think the best way to do this is to give in to everyones' demands. One of Aesop's Fables tells of an old man and his son bringing a donkey to market. At first the boy and the old man led the donkey.