We have come up wit ha workaround to fixing the spambot issue. We've halted all registrations, and replaced the sign up button with a link to a form.
New users can then email us a request for an invite link. I think this is a good way for us to the protect the instance, at least in the short term.
If any other admins who are having issues with bots want the code for our form and details of what we did, let me know.