[Visual77]

This will disable all notifications, MacGyver. Go to left4dead/cfg/sourcemod and open plugin.all4dead.cfg with notepad, change the following and save:
Whether or not we announce changes in game.
// -
// Default: "1"
a4d_notify_players "0"
//
If that dosen't work, add a4d_notify_players 0 to your server.cfg.