How can I limit user connections to folders in a dfs share?

Hello all!

I have had some issues with users having the same file open on a dfs share and dfs only accepting changes from the last user to close the file. the network is a windows 2003 domain with 2 DC’s one being SBS 2003 r2 and the other a 2003 R2 server.
How can I limit connections to folders in a DFS share? Or warn users that a folder or file is in use by another user?

Many thanks
Steve