1) Get used to it. It will not stop. 'several' users is small time. I get a dozen a day. Others get far more.
2) add a captcha.
3) require that you manually validate each new registration.
4) manually ban large swaths of Ip addresses - look at the IP's of the spammers. I blocked much of China, Ukraine, Pakistan and I think Indonesia and that shut the majority of it down.