If Quirrell wanted to prevent this, the simplest way would be to blackmail Harry
The simplest way would be not to help him. Harry can’t make portkeys to and from Azkaban himself, he can’t Disillusion himself once inside, fight random Aurors in the corridors (bet they’ve tightened security for a while), and now he can’t even leave Hogwarts against Dumbledore’s will.
Disillusionment isn’t a real problem, since Harry has a cloak, and I’m fairly confident he could find other ways to hide. Polyjuice might be another way, but it has some disadvantages. The upshot is that I would expect Harry to be able to pull it off, given enough time to prepare.
The simplest way would be not to help him. Harry can’t make portkeys to and from Azkaban himself, he can’t Disillusion himself once inside, fight random Aurors in the corridors (bet they’ve tightened security for a while), and now he can’t even leave Hogwarts against Dumbledore’s will.
Disillusionment isn’t a real problem, since Harry has a cloak, and I’m fairly confident he could find other ways to hide. Polyjuice might be another way, but it has some disadvantages. The upshot is that I would expect Harry to be able to pull it off, given enough time to prepare.