As Jan said, the easier method is not to share the file.

If you want to pursue the matter, I recommend first making an audit trail. You can add one via a worksheet, an external file like a text file or an Access MDB table. An example using the latter method is in this thread.
http://vbaexpress.com/forum/showthread.php?t=22620

It is then just a matter of checking the CloseDate field in the last record. If empty, then the user in that record has the file open. The Open event would check it and then add your username or show the message and close if open already.