Oh, that sucks.
I see the forum is running version 2.0.17 of the SMF package, whereas the latest patch just made available is 2.1.2.
Not suggesting it will specifically include any new anti-spam features, but it does mention security updates which sounds like a good thing.
Do you have to use Google captcha? From the SMF Security and Moderation page
Configure Verification Methods
Below you can set which anti-spam features you wish to have enabled whenever a user needs to verify they are a human. Note that the user will have to pass all verification so if you enable both a verification image and a question/answer test they need to complete both to proceed.
If you want users to answer verification questions in order to stop spam bots you should setup a number of questions in the table below. You should pick relatively simple questions; answers are not case sensitive, though you should not use a 0 (zero) or a space as an answer to a question. You may use BBC in the questions for formatting, to remove a question simply delete the contents of that line.
Visual verification image to display - This allows you to choose whether to add a verification image and to change its difficulty.
Number of verification questions a user must answer - Select any number above zero to set the number of verification questions which need to be answered.
It sounds like there are other user verification options available, including custom questions. Or are they already being used and simply don't work?
(It's a long
time since I registered, so I can't remember what I had to do)
SMF also supports Mods, including this one:Spam protection, AntiSpam, FireWall by CleanTalk (no Captcha/reCaptcha)https://custom.simplemachines.org/index.php?mod=3851
(Disclaimer: I've never heard of CleanTalk before now - I just found it while sniffing around for anything that might help)
It does require a CleanTalk account and paid license, but I see the price of a license for one year is only $8.
If it works like they say it does, that would seem like a good investment. Hell, I'd even pay for a year myself and I'm sure there are other forum members that could find eight bucks down the back of their couch.
Bee excellent to each other...