That's called visual conformation and yes, it's already implemented. People can sign up and there's not much that anyone can do about it. It primarily defers the bots but I think even now people have made bots smarter to be able and read some of these visual conformation things...least I think...that's crazy but...who knows.
Most spammers use bogus e-mail addresses so they never get activated but they can still show their web site link, which gets indexed by google, so unless I want to mod it that only active people show up, that's one more thing for them to cheat with. I personally think that it should be an option in phpBB. And actually thinking about it, it will be in the 3.0.x release.
I'm just waiting....it's going to be bomb!
Then I get to overhaul the forum and make a new style.
