How do I remove the ability for customers to sign up to our account?
To remove the option for your customers to sign up to your Help Center, you would need to disable the Anybody can submit tickets option.
Click the Admin icon () in the sidebar, then Customers > Settings. Please disable the option Anybody can submit tickets.
If you just want to hide the sign up button, see the article: How can I completely hide the sign up button from my Help Center?