This is the kind of stuff I say a lot, honestly. No matter what the situation, once you've said no unless something truly changes you need to stick with it. Because you're right; people accustomed to getting their way will try a bunch of manipulative tactics to get what they want, but if you keep telling them no and leave no room for argument they eventually either get tired/bored and stop or realize that you're not going to give them what they want and it's not worth their energy to try and keep it up.

It was hard for me to learn to say no to people, I have anxiety and was always nervous so I just said yes. But I found something when I started to say no; well, a few things actually. A sense of self-confidence I hadn't known before, and that the people I allowed into my life stopped being those trying to manipulate me and use me as a doormat and started being actual, genuine friends that wanted my well-being as much as I wanted theirs :)