Yes, there is. In the back-end, navigate to Users > User Manager. When loaded, click the options button from the sub-menu. The first option in the Component tab is a radio to allow or not allow user registration.
Now, when someone (or some bot) attempts to navigate directly to user registration they are automatically redirected to a dedicated log in page. Which, without an account is pretty much useless.
You could also inset 301 redirects into your root .htaccess for any URL paths and route them back to the home page.
Redirect 301 /index.php?option=com_users&view=registration http://www.mywebsite.com/