The KP_Divide, KP_Add and possibly other keys, at least when together with Ctrl (Primary) modifier, seems to be **unremovable**. If I "remove" the keybinding in Preferences ("Change", press nothing, click "OK"), they remain active. Neither ~/.config/geany/keybindings.conf nor ~/.config/geany/snippets.conf contains "KP_Divide", but pressing Primary+KP_Divide causes a zoom reset ("normal size").
Moreover, it seems that keybindings in **snippets.conf** are "inferior". If I do an explicite binding for Primary+KP_Divide in keybindings.conf then it overrides (removes) the "hardwired" (see above) zoom reset effect. However, when I do the binding in snippets.conf ("[Keybindings]", "comment=<Primary>KP_Divide"), then it does NOT override the "zoom reset": the effect is that *both* action is played. (Side-effect, merged effect.)
My environment is: recent Xubuntu (=Ubuntu with some xfce4). $ geany -V geany 1.28 (built on Sep 23 2016 with GTK 2.24.30, GLib 2.49.6)