After hundreds of debug processes, I've finally found the function which is used to do that. Here is screenshot:
I'm a novice develpoer, and I DO NOT WANT MY USERS TO ADD NEW TAGS. How can I do for it. Or can I delete some tags?
Search "tags" in the ACP search bar. There should be an option to manage tags and only allow certain ones.
@PitaJ is correct, the option is called the "tags whitelist", and you can decide which tags to allow, and do so on a per-category basis