How to remove add new tag field?

The simplest solution would be to just hide it with admin CSS. Knowledgeable users would be able to unhide it, but they’d first need to realize it’s missing and be motivated enough to check if it’s really gone or just hidden.

It’s possible to back up the hidden field server side by preventing certain roles from adding new tags. This isn’t too difficult, what’s more difficult is securely removing the field HTML from the editor.