How about-

Dear PITA:

it was a very difficult tax season and a few clients made it even more difficult.   While sitting in my office today and reflecting on this tax season, your name was the first to pop in my head.  I don’t care to have another repeat of this tax season, so please just don’t ever darken my doorway again.  Just go away.  And please don’t seek out another local tax preparer.  I would feel terrible to curse my local competitors with you.  Please seek a preparer from outside the area.  Actually, finding one in another state wouldn’t be such a bad idea.  

Respectfully,

Your former tax preparer