I was having a similar issue with spammers. What I did was install a mod called "Delete Unconfirmed" accounts (not sure how great it actually is as I havent fully tested) and also made the membership approve automatically.
I gave absolutely no rights the the new signups - Instead they have to upgrade to silver membership after signing up to post anything. (I added some language strings to the message after signup with a link to upgrade their free account to the next level and made it look like I was giving them a paid account) (real members)
That eliminated the autosignups/tools posting crap on the forum. I am not sure about the Delete Unconfirmed module/changes though as I haven fully tested as yet.
You could do something similar.
I will most likely buy a better captcha mod later?
Sorry couldn't be of more help.
The Delete Unconfirmed was free but now its $5. https://www.boonex.com/m/delete-unconfirmed-members
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