org-agenda-skip-function-global

org-agenda-skip-function-global is a variable defined in `org-agenda.el'.
Its value is

nil

Documentation:
Function to be called at each match during agenda construction.
If this function returns nil, the current match should not be skipped.
If the function decided to skip an agenda match, is must return the
buffer position from which the search should be continued.
This may also be a Lisp form, which will be evaluated.

This variable will be applied to every agenda match, including
tags/property searches and TODO lists. So try to make the test function
do its checking as efficiently as possible. To implement a skipping
condition just for specific agenda commands, use the variable
`org-agenda-skip-function' which can be set in the options section
of custom agenda commands.

You can customize this variable.