Philippe,
 
Most if not all of the display properties of the OTRS system may be modified by editing the various dtl files in the /opt/otrs/Kernel/Output/HTML/<Style>/ directory.
 
For example we remarked out the Company Tickets lines in the CustomerNavigationBar.dtl file as we didn't use that functionality.
 
Enjoy :-)
 
Brad West
Network/Systems Engineer
Spang & Company