Found a nice samba tip to prevent users from storing media files on a Samba server.
Just add to smb.conf in [share] options:
veto files = /*.mp3/*.wav/*.avi/*.mpg/ # you can continue the list
The veto files option separates file specifications with the slash character (/) and supports the standard wildcard characters, asterisk (*) and question mark (?).
What do you do if you want to enable this option for all shares? One option would be to place a veto directive in each share's configuration. But, a better option is to place your veto directive in the [global]section. This will have the same effect as placing the directive in each and every share