If by overcharging you mean billing for more hours than were in fact done, that's fraud and is a different matter entirely.

Obviously you may stop working with a person if their prices are too high or because quality of their work is shit.

However if you use this as a pretence for firing contractors, but the real reason you are firing them is because of their sex / gender / race - that would be illegal.

In the same vein, if you fire contractors for doing things that are none of your business, and it is common knowledge that you do that and exercise control you should not have over them, then a court may decide that in fact the relationship is not that of a contractor. That in fact this is nothing more than a legal fiction created for the purpose of tax avoidance or similar.

>In the same vein, if you fire contractors for doing things that are none of your business

Like what?

I'm pretty sure you can put all the terms you want in a contractual relationship. Even things that are none of your business (as long as not protected class).