We don't provide an option to entirely delete tags from the system.
In case you do not need a specific Tag List or Tag, you can archive it at any time. Archiving tags will not affect the already tracked time and will not remove tags assigned to your time entries.
Navigate to the Tags tab on the left side menu and click on the Archive icon next to the list or next to the specific tag to archive it.
You can reactivate them at any time if necessary. To do so click on the Show archived tags button and restore them with a Reactivation button.
Only the Administrator from the main parent group can manage and archive tags.