If a user wants to prevent other users making any changes to an entire stream the user can lock the stream. To do so right click on the stream and select 'Lock' from the menu.
An example of where stream locking might be useful is when submitting changes. In large projects after making some changes the developer may be required to 'Update To Latest' to integrate everyone else's changes. He/she may then be required to run many unit tests and acceptance tests before the changes can be submitted. It would be very frustrating if after doing all this another user had made another submit so the whole procedure needed to be repeated. In this situation the user might lock the stream before the 'Update to Latest' and unlock it after the submit.