You could add a CAPTCHA test to the registration form to prevent most of the spam accounts created. Try this plugin for that:
https://wordpress.org/extend/plugins/si ... wordpress/ I understand that it works with s2Member.
I've also found this other plugin to prevent spam registrations especifically:
https://wordpress.org/extend/plugins/st ... ns-plugin/ You will have to test how it plays with s2Member, though.
If the registration is only meant for paying members, though, you should disable free registrations from here
WP Admin -> General Options -> Open Registration / Free Subscribers, set it to
NO.
Hope this helps.