Six Apart has offered Typekey authentication in Movable Type since version 3.0 and it's is a great way to stop comment spam. However what if you don't want to (or can't) use it?
Comment Email Filter, a new plugin for Movable Type, allows you to trust or ban commenters based on their email addresses using an incredibly simple interface. You can go one step further with Comment Email Filter and only allow comments to be published if a trusted email address has been entered (a great way to nip unwanted comments in the bud!). The beauty of Comment Email Filter is that it lives on your server, you don't have to rely (or trust) a third party and best of all, you manage it!
All this is available through an easy to use interface with MT Enterprise users are being provided with a more verbose interface for listing, trusting and banning email addresses.
Movable Type versions: 4.3