I want to share with you our experience with blocking spam robots and guest spammers. We wanted to have our boards as open as possible and allow posting as guest without captcha. We also wanted to allow registration without captcha as the phpbb3 captcha is not bot safe.
We have achieved to block effectively all guest spammers and also all robotic registrations by installing the following MODS.
1. Anti Bot Registration. This MOD allows you to define the language used during registration that bots key on. It changes the variable used in the "Agree" button when agreeing to the board terms. This ia a very effective MOD. Link
2. No more spambots. This MOD Blocks registrations from timezone UTC - 12 (Baker Island Time) that is used by almost all spambots. Link
3. Authorize URL for Groups. With this MOD you can disallow guests to post something with a link or IP address (works in signatures and in PM's too). Link
4. Censor block. This MOD uses the word censor list to match and block any post, PM with such a word. So instead of replacing it is much more effective. Link
. We have a long list of about 240 words and this MOD works perfectly.
With the above four MODs you can open your board to guest users and disable any captcha (making your board easy accessable for all). We've not had one post come through (for now).
Hope this is helpful information.