If you delete the Desktop[ALP] service you will not be able to search the GAL. It's like trying to send email without the Desktop[CMIME] service book, it just doesn't work.
This is a feature because most companies, and users, rely on being able to lookup a name that they don't currently have on their BB.
Having said that, you should be able to disable this feature. You can get the IT Policy guide from BlackBerry Technical Solution Center
There should be a Product Documentation link and you will find the guide in the BES section.
It's easier then looking at the Policies on the BES.