Beware!
Applying captchas to tmo will make you share the same little space in hell with all operators of captive portal hotspots

Anyway, the problem is we are propably dealing with human spam monkeys that can pass captchas.

A social attempt could be more effective.
Newly created accounts should be read-only and delete themselfes one week after creation if not activated.
To activate the account, the new member has to write a two sentence "Hello i am using device X... and am here to..." selfpresentation PM to Councel or Admin.

It will be relatively easy to identify a spammer that goes the length of attempting to get an activation by lying on his motives. My guess is they do not even know what tmo is all about and will gobble something like "Helo Friend, I legit, please activate!"

Maybe a invite system could also work where trusted members can sent invite tokens for auto activated new accounts.

Granted i have no idea how complex this would be to implement into the current system.
Just my humble idea to stop spammers for good.