To turn off bezels for this user until next login:

launchctl unload -F /System/Library/LaunchAgents/com.apple.BezelUI.plist

To undo, change unload to load.

To turn off bezels indefinitely for this user:

launchctl unload -wF /System/Library/LaunchAgents/com.apple.BezelUI.plist

To turn off bezels indefinitely for all users:

sudo defaults write /System/Library/LaunchAgents/com.apple.BezelUI Disabled –bool YES

To undo, change YES to NO, or:

sudo defaults delete /System/Library/LaunchAgents/com.apple.BezelUI Disabled