Disable default option key binding

Solution 1:

Save the following to ~/Library/KeyBindings/DefaultKeyBinding.dict (create if necessary).

{ "~a" = (); "~b" = (); "~c" = (); "~d" = (); "~e" = (); "~f" = (); "~g" = (); "~h" = (); "~i" = (); "~j" = (); "~k" = (); "~l" = (); "~m" = (); "~n" = (); "~o" = (); "~p" = (); "~q" = (); "~r" = (); "~s" = (); "~t" = (); "~u" = (); "~v" = (); "~w" = (); "~x" = (); "~y" = (); "~z" = (); }

This disables all the ⌥<letter> combinations. Restart apps to take effect.

The DefaultKeyBinding.dict above is an old-style property list¹, defining key-value pairs using the syntax { key = value }. In this case, the keys are the key combinations to activate the method in the value.

Standard dictionary key symbols are used: ~ alt, ^ control, $ shift, @ command.
Therefore ~a means ⌥A (alt-A) and ^$1 would be ⌃⇧1 (control-shift-1).

The value for each of the keys is set to ()—empty parentheses without a method. As DefaultKeyBinding.dict overrides /S*/L*/Fr*/AppKit.*/R*/StandardKeyBinding.dict, the shortcut is defined to be without method and therefore does nothing. It's preferable to create your own key bindings file like this rather than editing the system one.

^{1 Feel free to use a new XML property list style if you prefer—it works the same.}

Solution 2:

The solution also might be to use different keyboard layout, e.g. U.S.:

• problems with "alt" keys: U.S. International, ABC
• solution: U.S.

Change it in System Preferences -> Keyboard -> Input Sources.

I found the solution on Stack Overflow here so you might give the author a credit if it helped you also ;)