I basically do this already but specifically for players who have the default name "Player". You should be able to modify it as needed. You'll have to change the logic for preventing a name change because I only disallow "Player" but it shouldn't be that hard.
https://github.com/fysiks1/amxx-noplayername
There are other examples out there for preventing name changes if you search for them. Might need to use an actual search engine though since you can't search for the term "player" on this forum (because it's too popular a term).