org-agenda-auto-exclude-function

org-agenda-auto-exclude-function is a variable defined in `org-agenda.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
A function called with a tag to decide if it is filtered on '/ RET'.
The sole argument to the function, which is called once for each
possible tag, is a string giving the name of the tag. The
function should return either nil if the tag should be included
as normal, or "-" to exclude the tag.
Note that for the purpose of tag filtering, only the lower-case version of
all tags will be considered, so that this function will only ever see
the lower-case version of all tags.

You can customize this variable.