Do you want those values to not appear in the graph at all, or do you just not want the user to be able to individually filter on them?
the departments that i want to remove from the list should not appear on the lists at all.
this makes me think i could exclude them on the query for the data set. but forgetting that for the moment i thought there was a way i could do this easily from one of the menus in tableau.
There is definitely an easy way to do it in Tableau, through the use of a context filter. Create a calculated field that called "Department Context" with the code
Drag this field onto your filters pane. Right click "Department Context" in the filters pane -> Select Add to Context.
Now, right-click the arrow in the top-right corner of the "Department" quick filter -> Select "Only values in context".
This should work. If you need me to mock it up for you, I can do that tomorrow when I have access to Tableau again. If you are unfamiliar with context filters, I have a short blog post about them. http://breaking-bi.blogspot.com/2013/01/types-of-filters-in-tableau.html
that did the trick.