WordPress disable /wp-admin registration form

I’m working on a site with a plugin that allows users to register in the sidebar with a CAPTCHA. However if you go to /wp-admin you can still register without a CAPTCHA, is there any way to disable the registration form when someone visits /wp-admin without disabling registration for the whole site so the sidebar registration widget still works?

To be clear I want to remove the form from /wp-admin not just disable registration for the site.