Why my text is deleted when I pressed Alt-Z?

I've annoying problem.
Each time when I'm pressing Alt-Z (not Control-Z), its doing Undo. It should be for 'ż' char. I didn't have this problem with TInyMCE.
How I can disable it?
I've read that I need to disable custom_undo_redo_keyboard_shortcuts option, but I don't know where.