I personally killed them all off, no way in heck are they going to blackmail me out of 5K. Bloody males in dresses, calling 'em robes.. I can now cast freely any spell in Amn, without having spent a dime, and no one is telling me not to do it
However, note please, that this fight is better to take on at later stagest, when chars are at higher levels. But yes, there's a dude in the Council building who will "sell" you a licence for 5K.
Also, I suggest you check out the
Baldurash page, there are many bugfixes regarding BG2: SoA. Namely many items in the game (i.e. some Keldorn's gear) will make the Cowled Fools.. er Wizards, to come after you, because mistakenly the game takes that as a spell, cast by a wizard. Apply them (basically just copy the file to override folder) and many issues should be fixed for you.