Hustle - receiving many spam mail registrations

Hi
To allow visitors to sign up to the blog, I have installed Hustle, so it integrate swith Mailchimp. I have been receiving new registrations from spam emails daily.
They do not appear in the Mailchimp database by the way. Only as 'users' when I login to my Wordpress site.

Any advise what I can do about this?

Thanks and kind regards
Ellen