i think the result from this RBL websites is worse than using CAPTCHA. they just not really reliable.
most users in the web have not valid IP
eg: in irxoops.org it recognize me as a spammer

hi irmtfan.

This is just an option.
And the method of IP BlackList might be useful for another environment like Japan.

yes, we should follow all possibilities.
i am very appreciated for your other plugin "js insert check" that prevent spammers from register. its result is fantastic.

i just decided to use CAPTCHA if any other options can not help the situation.

i see CAPTCHA is just useful when your site is under attack by spammers that customize themselves for the site input forms. also in that case i prefer to block that IPs and create my own blacklist rather than using RBL.

I respect your choice

It might be easy to create CAPTCHA filter for Protector.
If I have a vacant time enough, I'll make it.