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 ⌥ 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.