Tags are just a text file with a name and the sha of the tag object (with the commit and some metadata/signatures as contents), last I checked. It's deliberately simple and thus almost impossible to actually lock it down in concrete terms.

Packed refs are a little more complicated but all of the storage formats in git are trivial to manually edit or write a tool to handle, in extremis.

That's the purpose of the forge platform, to provide a way to prevent changes to these files from being accept into the source repository. For example:

https://docs.github.com/en/repositories/configuring-branches...

https://docs.gitlab.com/user/project/protected_tags/

https://forgejo.org/docs/latest/user/protection/#protected-t...