Does anyone know how to control a vocabulary using tags?

When we create a controlled vocabulary using tags is there a way to lock that down in custom tag fields for the people doing the tagging? We have set up a robust controlled vocabulary using tags. We have also created multiple custom fields and mapped each one to the specific tag string it pertains to. I want to limit people doing the tagging to only being able to use the existing tags in the specific string thats mapped to the field, however, even though the field is mapped to those specific tags I can’t find a way to prevent people from free typing a tag into the field. Is there a way to lock that down so people can't free type anything in there? If people need a new tag I'd like that to only be able to happen through a request to the admin.