I am not sure how they are getting thru it has to be a real person is actually registering on the site - no way it could be a bot, but they register, verify, make it thru the security questions and captcha, upload avatar / and background photo and start making posts with scantily clad women offering all sorts of escort or "other" services.

any advice?

lol, I don't think I've had any from India lately - seems to be Russia and Spain.  But mine are just using the Contact Us form - not registering.

 

There are a lot of services, where human-employees works for create accounts. Then they sell it to the spam scripts which do their jobs. It's really exist. In that case you should think better way especially for you community (not block good users and make spam-employees cost for register too expensive then they sell it). So, for example, you can set premoderation for users, who posted less than 5 posts at all. After reaching this level they are can use forums as others. This example way is more expensive for create 'white' user with approved 5 good posts. And it will be cool for you - 5 good posts is good cost if this user will get permanent for 1 spam (5 good posts are still exist and make your forums better)