If you have the registration disabled, i’m sorry, but it looks like an infection of your site and the spammers doesn’t need to pass by your registration form.

Remember if you have installed any plugin (nulled). If yes, delete it, coz generally are infected.

Try to update all your plugins, wordpress… put a captcha to your registration form: you can search of a plugin or you can see this tutorial with a plugin that i found: https://registrationmagic.com/add-captcha-wordpress-login-registration-form/

If the problem persist, (or not), it will be good to scan your site, there is some good plugins as Wordfence.

Good luck!