Disclaimer: I have never been in a real position of "having clients" as I have never done contract work or similar, so take what I say with that in mind...<p>You have essentially a couple of options. You can be blunt:<p>"Go away. I don't want your business any longer. Don't call back."<p>...or you can be tactful:<p>"I am sorry, but I have decided that I am unable to continue to fulfill your needs as a client, and so I am terminating our business relationship."<p>Expand either however you feel is necessary. Note that being blunt should probably be reserved if being tactful fails to work adequately.<p>Being tactful can be made less painful for the client if you are able to both gracefully exit from the relationship, while also potentially giving them suggestions on how they can continue to address their needs outside of your business.<p>For instance, if you are able to suggest a complementary service, or even a competitor to them who might be able to better address or support them, do so (this works even better if you are on speaking terms or otherwise engaged with those businesses; they very well might refer clients back to you as well).<p>The client won't like that you are dropping them, but the fact that you are referring them elsewhere will leave a professional and good impression in their mind that even in the process of terminating the relationship, you suggested support for the transition.<p>Lastly - both of these will need to modified - if they can be used at all - depending on what kind of contracts and agreements you already have in place with the client.
You charge them for the chasing, for the time spent to nail down the speculative, for the time spent on the changing requirements, and so on. You charge them enough to be worth it, or enough that they go away.
The way that works best in my opinion is to over communicate and lock them in on a decision. Once that happens things like requirements/scope changes cannot happen as you already established this in writing and it will be binding.<p>For instance for any work outside of the scope, you politely inform them of the service of work that they agreed to and that any other work outside of that will cost them money because it takes up your time. These are standard and should be outlined before you do a thing with/for anyone.<p>The clients importance will determine if you adhere to what I posted above. If they are toxic as you defined, when you have those initial conversations and contract negotiations they will be put off by the rigidity and not want to work with you :).