It's difficult to prevent users from creating throw away accounts. The obvious email providers as you mentioned can be banned, but then this leaves the trash that is posted from accounts such as gmail.
This makes me think that this suggestion Can first posts be reviewed before being becoming visible on the site? might be the way to go, having a lower rep requirement to review such posts. An example of 500. Although this would slow down post becoming visible on the site, it would prevent garbage also being visible on the site. If this garbage never becomes visible to the general public or lower rep users, then there will be less joy for such users to create throw away accounts, as their questions will never find answers.
It would be interesting to see the stats on such a change on the efforts of spammers and trolls, as if they cannot view their public handywork, it may slow down their efforts, again it may not.
I think it would be worth a temporary trial to test such a system and see if the community can cope with the review queue.
Account registration for answers could also be enforced.
Looking at the statsShog has posted:
Total deleted, closed or negative score questions of users < 10 rep:
33.92+26.79% = 60.71 60.71 * 108664 = 66091
Total deleted, closed or negative score of all questions:
24.22 + 20.27 = 44.49 276792 * 44.49 = 123145
Total deleted, closed or negative score of all questions of users > 10 rep:
123145 - 66091 = 57053
20.61% of all questions asked by users greater than 10 rep are deleted, closed or downvoted.
Can we determine if there is a minimum rep point where this percentage drops significantly?
Reviewing posts of users < 10 rep before making them visible would significantly reduce close and delete review queues, as posts that are not made visible would not need to be vetted.