Look in the smb.conf manpage and search for "veto files".
I think you would use "veto files = /*.mp3/*.avi/*.mov/" to block mp3, avi and mov files. A person could get around it by removing the extension.
Here is the Samba 3 Howto & Reference Guide.
http://www.samba.org/samba/docs/Samb...Collection.pdf
Alway run "testparm" after making manual edits to smb.conf. It with point out any errors in your configuration file.