Untagging a file or another object is as simple as deleting it. Don't worry: an object is actually deleted from Tagsistant only when it's removed from its last tag. As an example consider this situation:
$ cp /some/file.txt ~/myfiles/store/docs/texts/@/
$ rm ~/myfiles/store/docs/@/file.txt
Here file.txt has been untagged from docs but it's still recorded in the database and tagged as texts.
Deleting a tag is something very simple to do from the command line. A tag can be deleted using rmdir from the store/ or the tags/ directory:
$ rmdir ~/myfiles/store/useless_tag/
$ rmdir ~/myfiles/tags/another_useless_tag/
However when using a filemanager you are forced to delete the tag from the tags/ directory. This is mainly the reason why the tags/ directory exists. Under store/ each tag contains all the other tags and operators to let you build your query path:
$ ls -l ~/myfiles/store/jazz/
dr-xr-xr-x 1 tx0 tx0 140K Dec 19 00:14 -
drwxr-xr-x 4 tx0 tx0 140K Dec 19 00:14 @
drwxr-xr-x 4 tx0 tx0 140K Dec 19 00:14 @@
dr-xr-xr-x 1 tx0 tx0 140K Dec 19 00:14 +
drwxr-xr-x 4 tx0 tx0 140K Dec 19 00:14 =maiden_videos
drwxr-xr-x 4 tx0 tx0 140K Dec 19 00:14 iron_maiden
drwxr-xr-x 4 tx0 tx0 140K Dec 19 00:14 music
drwxr-xr-x 4 tx0 tx0 140K Dec 19 00:14 file:
drwxr-xr-x 4 tx0 tx0 140K Dec 19 00:14 time:
A file manager would probably refuse to delete a non empty directory or would ask the user to delete it RECURSIVELY! Of course this would WIPE OUT YOUR WHOLE REPOSITORY. Please pay a lot of attention when deleting something from the store/ directory and NEVER DELETE A store/ INCOMPLETE PATH. A path is incomplete when it does not include a @/ or @@/ mark.
To let you safely manage your tags without risking your entire repository, use the tags/ directory. Under the tags/ directory, tags have no contents. If you delete a tag from the tags/ directory you just delete that tag and nothing more, without affecting the rest of the repository.