Uncheck the ‘visual bell’ box in the preferences for your terminal style.
This is probably not the right place to ask this question. But there is a setting for that: Settings/Advanced/Visual bell
Step (1) – Disable Visual Bell
Terminal > Preferences > Profiles > Advanced > Bell
Uncheck the “Visual Bell”
Step (2) – When Audible Bell is still enabled and screen continues to flash
System Preferences > Accessibilty > Audio
Uncheck the “Flash screen when alert sound occurs”
If anyone is using iTerm2 and having this problem, try going into Preferences->Profiles->Terminal and check “Silence Bell”.
If using X-term, Hold down CTRL. Press and hold the MiddleMouse. Move to ‘Enable Visual Bell’ and release the mouse button. This would make a great community wiki…how to do it in every terminal!