During development I defined an 'initialize-instance :after' method which after a while was not needed anymore and actually gets in my way because inside it calls code that is not valid anymore. Since the unintern function does not have an argument for the qualifier, is there any way I can "unintern" the symbol-qualifier combination of a method so that I don't have to slime-restart-inferior-lisp and load the project again from the start?
You can use the standard functions find-method and remove-method to do it:
(remove-method (find-method #'frob '(:before) '(vehicle t)))
I find it's much easier to use the slime inspector. If your function is named frob, you can use M-x slime-inspect #'frob RET to see a list of all methods on frob and select individual methods for removal.