Make sure the access rights are limited to the certain groups you want them for. Usually this means removing them from the groups that already had them (prob. stock and sales).

Go to settings --> Users --> Groups and open up any group. On the tab "Access rights" you willd find a collection of objects and their CRUD-settings for that particular group. Uncheck the "write / create /delete" options from the groups you do not want them under.

Under the group for administrators (if that exists), add the line for Unit of Measurement object and check all the boxes. The last step is to just make sure that all your users are added to the right groups.