Most of the free e-mail services (i.e., throw-away e-mail addresses) are blocked from registering.  An error message pops up if you try to use one of the blocked domains, so there shouldn't be any confusion on that score.

While I'm sure it's a hassle for many people, it was a bigger hassle for me to deal with the problem children who enjoy using anonymous e-mails to cover their tracks.  Everyone who has an ISP also has a corresponding e-mail address provided by that ISP -- they can use that if they'd like to register.

Regards,

Foul