Unless user registration an integral part of your plugin, why not disable it till the time you're ready for it?

Alternatively, I think you could try restricting the usernames you can use on your website - that is, users can only use a few usernames that you define if they want to register. I'm speaking by looking at the piece of code on this website -

https://sozot.com/wordpress-username...hout-a-plugin/

This code actually blocks certain usernames from being used, but try just reversing the functions after the double ampersands (&&) and that could do the trick for you.

(There is also a plugin called "Restrict Usernames" which has an option where you could mandate that usernames should be of a certain format, that could serve your purpose. But keep in mind that it hasn't been updated in an year, so not sure if that would help).