ring fence
noun /ˈrɪŋ fens/
  /ˈrɪŋ fens/
- (finance) a form of protection that stops a sum of money from being spent on anything other than its intended purpose - The government has promised to put a ring fence around funding for education.
 
- a form of protection that only allows particular people to use or know something, or only allows something to be used for a particular purpose - He did not say when the merger would be completed or when the ring fence would be abolished.