As I understand it, to gain the "Use Grimoire" skill, you need to break through to the Grimoire's node. This is a 10W6 barrier and therefore completely impossible to most people. You accomplish it as a graduating apprentice through community support and the benefit of your school's pre-prepared rooms and the like.
To buy a spell uniquely, you have to break the 10W3 barrier around a spell node. That attunes your talisman. Those "extra" spells schools know that are not part of the grimoire require this as well. (Oh, and spells don't ever add to the Use Grimoire ability. They aren't like feats, where you buy a bonus to the affinity rating, you buy individual spills completely separately)
If a wizard managed to get his hands on the grimoire of another school, he could attune himself to it if he could break that 10W6 resistance. (Not an easy task, even with community support, since I presume many of the aiding rituals are node specific.)
He could also learn spells from that Grimoire individually by breaking 10W3 barriers. (Easier than attuning a grimoire.)
An adept who leaves is Banned. This Ban spell severs the attunement that they already had with a grimoire, and specifically makes access to any of the school's nodes more difficult. Thus if ever the talismans he already has are broken or lost, he loses those spells as well. If he still has a physical copy of the Grimoire, he can re-attune it by breaking a 10W6 or create new talismans with 10W3, except that if he does he also gets hit by whatever it is that the Ban does. (Which is, presumably, very nasty.)
So why can't you curse an Illuminated person?
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