Word similar to "farewell" but with negative connotation
Good riddance.
I'm done with you. (If you are. :-) )
For that matter, if this is in writing, "goodbye" may do what you want. It's pretty rare to close email or a letter that way, so this suggests closure. If you're speaking orally, then as drm65 says you can use tone of voice to convey your dissatisfaction.
It is easy to say "good-bye" as if you meant "good riddance!" Try it!
Good-bye doesn't necessarily mean I wish you well anymore.