You are free to create a plugin combining email and password field types.

I personally hate fields asked twice (except for password-type fields where you do not see what you type so it's understandable). Always using copy-paste after proof-reading, thinking "this site thinks i'm an idiot", which isn't positive.

Also someone knowingfully entering wrong email, will enter it twice too, no worries.

Way better "protection" is ajax tester + a clear "field delimiter" text above or below email address telling they better enter a correct address to be able to activate.

Then those who don't want to give address will create a throw-away email for single use.

You simply can not force people not wanting to.

Only way to filter is to make them pay....so there is a chance that they are interested (see CBSubs for that, for CB 1.2).