Employee who drives me nuts!

[Blankity Blank]

One other thing to consider is that maybe you suck at coaching. Teaching/coaching/mentoring is a skill like anything else and some are better at it than others. It's no sin to not be good at it. It could be anything from you suck in general at it or, maybe you don't, but, for whatever reason, you and this "student" just don't connect.

You've got a pretty damaged relationship with this employee. Rightly or wrongly (To be fair, we do only have one side of the story) your opinion of her is in the dumper (And don't assume she doesn't realize what you think of her, including the "... she isn't the brightest bulb in the marquee;" business), two botched firings, and now losing your cool and yelling at her (If it was in front of customers and/or co-workers double the damage done).

If your hands truly are tied, is there someone else, another supervisor or maybe senior employee, that you can let have a shot at coaching her?

As for making her life miserable, I'd be very careful playing that game, particularly with the history you two have already. The phrase "hostile work environment" comes to mind. Even if others have gotten away with it, under who knows what exact circumstances, it doesn't mean you will.