In Opera profile (see the path in opera:about), in the file Preferences, add or modify the node "Keybindings" to re-define the keybindings for + and -. Backup the file first!
"Keybindings": { "Basic": { "ZoomIn" : ["Ctrl+OemPlus", "Ctrl+Shift+OemPlus", "Shift+OemPlus", "Ctrl+Plus"], "ZoomOut" : ["Ctrl+OemMinus", "Ctrl+Minus"] } }(These are the default keybindings, except that I've removed "Plus", "OemPlus" and "Minus", "OemMinus".)