A tag is a name attached to a particular revision during the history of the project, and can be used as a human-readable alias to refer to a specific state of the source code. To be useful as such, the tag should be immutable, otherwise what now is tagged as the "1.24.0" release in the Pango repository may point to a totally different tree at a later date. For these reasons, deleting and updating tags are disallowed in GNOME git repositories.
The exact error message you get if you try to delete or update a tag in GNOME repositories is:
You are trying to delete the tag '$tagname'. http://live.gnome.org/Git/Help/TagUpdates
When importing a new project history to GNOME git server however, it is allowed to update tags, to clean up old cruft, but after the import phase, this is strictly disallowed. If you push a tag by mistake, just push a new tag with a new name with the correct commit.