Eliminate their position.
There is an even better way. It avoids any litigation, etc.
You redefine their jobs so they are nothing more than clerks doing government work. Don’t sent them to get coffee for their boss, that’s personal, but call them in every time you want one xerox copy made. Continually send them on “snipe hunts” for nonexistent files, and be very critical when they can’t find it. Send them to walk to building two or three blocks away to deliver a memo that could much more easily been sent by email. A memos like: “Bob, in response to your question, the answer is very definitely NO.” Please do not discuss this matter with the bearer of this memo, as they are not cleared for this.” Then put memo in an unsealed envelope. Of course they will read it!
Call everyone in for a meeting, discuss one or two unimportant items. Then tell the idiot to leave and close door on way out for rest of meeting.
At first, one other person will think it’s funny how the bad employee is putting one over on the boss.... then they get the same treatment. It stops right there.
They become laughing stocks at work, their resume is FUBAR, and quit within 3 months.