Portal - Don't show specific module on tab (even tho there is read access to that module)

Portal - Don't show specific module on tab (even tho there is read access to that module)

Hi,
Portal users see all the modules on the tab (top of screen - left to right).
I do NOT want my portal users to see a specific module on the tab (but they must have read access to this module).

It is possible to control what appears on the top of the screen tabs?  Or is this just related to whether the portal has read access??