You can disable the Unique Visitors section similar to how it was done in Urchin 3. Urchin 4.0 - 4.1 uses the same javascript engine that Urchin 3 did. However, be advised that this system is likely to change down the road, but we will make sure there is always a way to disable reports. Also, you will need to redo your modification after any upgrades.
To disable, the Unique Visitors section, edit the file:
lib/ugroups/default/navigation.js
located in the urchin installation. Uncomment or create a line that says:
parent.disable(1900);