You might try my IP_NoRegister hack here:
https://vborg.vbsupport.ru/showthread.php?t=129280
It works at the IP or the IP host level (assuming the host is something you don't mind blocking)
I honestly don't use it but I put it together from someone that PMed me saying they had the same kinds of problems you are having.
You can also move the plugging to a "New Thread" hook and then these people could register but not post. Personally, I would not care if I didn't have members with those IPs so I would just lock them out!
Good Luck!