How to disallow users from changing their emails? I need this because it makes life a bit more difficult for 'professional' spammers who just change their emails after logging in, so that the can use that very email for the next registration, after being blocked. 

Appreciate your hints.
You can either force users to confirm email change or protect the email field from changes.
Both modules have D6/7 versions. Second module can lock password and some other fields as well.