If you don't want to support concurrent development for some users then disable the 'Multiple Checkouts' policy as described in the section called “Defining a Policyset”. When a user edits a file the file will be automatically locked. When another user attempts to edit the file he/she will be unable to 'checkout' the file until the lock has been released.
The advantage of this is that the user will never have to merge another user's changes with their own. The disadvantage is that users will be unable to work on a file while another user is working on the file.