org-tag-persistent-alist

org-tag-persistent-alist is a variable defined in `org.el'.
Its value is

nil

Documentation:
List of tags that will always appear in all Org-mode files.
This is in addition to any in buffer settings or customizations
of `org-tag-alist'.
When this list is nil, Org-mode will base TAG input on `org-tag-alist'.
The value of this variable is an alist, the car of each entry must be a
keyword as a string, the cdr may be a character that is used to select
that tag through the fast-tag-selection interface.
See the manual for details.
To disable these tags on a per-file basis, insert anywhere in the file:
#+STARTUP: noptag

You can customize this variable.