Don't be so sure about that. Most captcha monkeys are "script" monkeys.
OCR reading is getting better, vbulletin AND phpbb captcha is routinely defeated with scripts out of the box...
Now, adding a humanizer question to the log in process (lots of hacks for boards do this) like: The sky is ____ (type the word blue)
The questions are easy and RANDOM. There is no scripting defeat for this so far.
I implemented the above when spammers were starting to filter through my captcha and it ended it stone cold dead.
I used to think like you, that these few that got through were manual entries, but they are not.
Spammers are inhertantly lazy. They will not lift a finger if a script will do the job. Your site to much hassle to script? They just forget you and move on. Their concern is to target and succeed with getting into the MAJORITY of the boards.
Blocking URLs is also a very good idea, congrats. The safest board from spammers are the boards that use multiple security measures. Combining captcha, humanizing questions and your URL hack will likely make your board spam free for many many years.