"Tags can be applied by authors and site visitors. Irrespective of their creator, all forms of tags are made available for selection, both when assigning to a page, or when searching."
- source: https://experienceleague.adobe.com/docs/experience-manager-65/administering/contentmanagement/tags.html?lang=en#deleting-tags
I have a technique to avoid selecting unintended tags:
- change your tag field on page properties to a selection field instead typing/choosing a tag. (simple)
- Assign read/write permission to the specific tags to a certain user group, so either anonymous group or even author group couldn't select them or use them.(recommended)
- Write a listener which looks for tagging events and validate the tags – remove/keep them accordingly with tag manager api (Not worth of putting that effort in my honest opinion)