It seems we can disable it after all:
To hide a Tracker as a project administrator:
Click on the "Tracker" link for the project.
Click on the "Admin" link.
Click on the name of the desired Tracker.
Click on the "Update Preferences" link.
Change the "Visibility of this Tracker" pulldown menu value as desired.
Default is "Visible to all site users". A change to "Visible to project
members only" will restrict view of the Tracker to members of your project
team. A change to "Not visible (hidden)" will prevent view of the Tracker
by any site user, including members of your project team; only Tracker
admins and project admins will then be able to see this Tracker when in
the admin view.
Click on the "SUBMIT" button to complete the change.
(thanks to Micheal Hudson)
-- Nikodemus Schemer: "Buddha is small, clean, and serious."
Lispnik: "Buddha is big, has hairy armpits, and laughs."