At this time the group administrator could enable and allow certain users to enter free form tags, but no to block the use of tags.

###Tags

Tags are special words used to associate topics with subject areas. Tags are created by administrators (or members with the "enter free form tags" permission) for the group and then applied by members to individual topics. For example, an administrator of a group called "Smart Phones" might create a tag called "android." Group members can assign this tag to topics in the group to identify the subject as relating to "android." Other group members can search on topics with the tag "android" to find related topics.

References
Enable and create tags for a group - Groups Help
Permissions - Groups Help

At this time the group administrator could enable and allow certain users to enter free form tags, but no to block the use of tags.

Tags

Tags are special words used to associate topics with subject areas. Tags are created by administrators (or members with the "enter free form tags" permission) for the group and then applied by members to individual topics. For example, an administrator of a group called "Smart Phones" might create a tag called "android." Group members can assign this tag to topics in the group to identify the subject as relating to "android." Other group members can search on topics with the tag "android" to find related topics.

References
Enable and create tags for a group - Groups Help
Permissions - Groups Help