Yeah, idea removing spells from staff sounds good, but maybe there can be also some other ways too. Here's my suggestion:
If player doesn't use the spells that he holds in his staff the percents of those spells would reduce after some time.
Like people tend to have disruption staves with commonly 1. magic missile 2. regrowth 3. disruption 4. you_name_it spells there, they just add those piece of crap spells to get 5 level disruption and 5 level from those additional spells.
So the idea would be clear: If you don't use other spells than Disruption, your other crap spells would drop and when they reach 0% they can actually lose level.
Also if someone has accidentally casted a wrong spell to his staff, he can just wait untill those crappy few percents goes to 0% and the spell disappers, leaving an empty slot for the spell he actually wanted.
Yours Seitan