I would start first by updating your forum software as the version you're running will be 4 years old next month!
That said, unless you just switched your verification method to ReCAPTCHA - then none of your readers have been able to register on your forums since the version of ReCAPTCHA running on your nearly 4 old version of Xenforo was retired at the end of March of this year (updating Xenforo would have fixed that).
Then either enable ReCAPTCHA or configure you site to run the Q&A verification and make your questions related to your niche.
In addition, using the CloudFlare Anti-Spam addon by
@Brent W along with Cloudflare will allow you to filter or block known spam countries.