Don't allow breaking exams by trolls

[hodak]

Currently each person that receives a link to an exam, has an access to edit it - I think it's nice because whole group of people can collaborate on an exam.

But, unfortunately, there's a risk of trolls removing questions, answers, changing answers to be invalid etc.

It's hard issue to resolve, do you have any thoughts on how it could be done?
One idea I have is versioning - each exam change would be saved as a "revision" and signed by user name. It should be possible to compare revisions and rolling back changes. What do you think?

Also, additionally, maybe users should have the ability to "ban" troll from editing the exam - for example vote from 3 different users would ban editing for another user.
What if voters were trolls and they banned everybody?
Hard things ;)

Maybe the user out-voted by 3 different people would lose access to exam, but would get this exam copied into his account?