You offer some tags to lower trustlevels to use, but not create — that is the most common way, I think. And let higher/moderators/staff create new ones when needed.
Or you let users create tags as they will, and then hight tl/moderators/staff fix tags if needed. On my forum this is in use, because creating tags isn’t an issue at all, but not use those is. So, low barrier.
Problem solved 