This is easy actually. Most states have "at will" employment. And even though you have "protected classes" of people , if you keep the issues strictly about job

This is easy actually. Most states have "at will" employment. And even though you have "protected classes" of people , if you keep the issues strictly about job performance and keep written documentation, it's pretty easy and pain-free to let someone go. Manager tells employee to do something. Employee refuses. Usually this falls under "insubordination." Manager tells employee to learn something new. Employee doesn't. This falls under "Employee is unable to perform duties assigned at an acceptable level." Without talking about age, race, ethnicity, etc, this looks pretty cut and dry - person couldn't fulfill the duties they were hired to perform. Of course anyone can sue for anything. But if you leave it as fact-based and about the work, not the person, it's pretty easy to avoid those issues. [edited for typos]