I’m a direct person, so I might say something to someone like, “I recognize that you might think that you need to do [x-behavior] in order to get attention and support; you have probably had to do that in the past to get service. I need you to understand that this won’t be successful in our working relationship and that my business will not be beholden to [x,y,z]. If we are to continue working together, we need to have very clear communication about what services are in and out of scope that we can both agree to. If we can do that, I’m happy to keep working with you and will do my best to support your needs. If we can’t, I’m happy to refer you to some places that might be a better fit.”
Raising prices should not be a passive-aggressive move. It should be to a price sufficient to offset the downside the customer brings (perhaps by hiring dedicated staff to deal with them), not to scare them off (it may have the latter effect, which is okay, but that's not the goal.)
If you aren't happy with them saying yes at the price you've set, you've set it to low.
I totally hear you and this fear stopped us from charging extra from a toxic customer. "What if they said yes?"