"More work for commenters" to type in a string of letters, maybe. But lots less work than erasing hundreds of emails.
Anything you need to do is fine with me.
I am in favor of Capcha if it works. I am in favor of requiring registration if it works (it can't be automated, though. People will just automate the registration process to spam).
One other alternative I have seen used is to take the Post button away and just leave the preview button. Then the user types his/her comment and clicks Preview. They see the preview and from there they can press Post.
I'm sure someone will figure out a way to make spam bots get around 'Forced Preview', but it might work for a while and is probably the least intrusive mode. (Plus it should help cut down on mispellings.)
Registration is one solid solution, the other being the captcha. A captcha is an image containing numbers or letters which must be retyped in a corresponding entry field. Because it is an image and not text, bots typically cannot recognize the text and it usually foils their attempts to post comment spam (although it does not help against trackback spam).
It means more work for commenters but is definitely less intrusive than registration. Registration works too; it does have the benefit of creating a community of readers/commenters along with the disadvantage of disuading the casual commenter.