How to restrict some tags for admin only?

Hi,

how to block others authors from using certain tags, but make them enabled when entered by admin? I need to create a list of few tags, which can be used by admin only. Those tags should be visible to other users (and searchable) but none but admin could add those tags to posts.
I thought it should be easy, as there should be some way to block nasty words from entering as tags, but I didn’t found any plugin, that would do this job and actually work.

Any ideas?

regards

Piotr