In the default vim emulation, vi-decrement is bound to Ctrl-X. You can go to Tools -> Options -> Keyboard and Mouse -> Key Bindings, and type "vi-decrement" into the "Seach by command" box to see where it is bound. You can remove the binding there.
I removed vi-decrement for my personal bindings, I would accidentally hit Ctrl-X in the window sometimes, and it was annoying if I didn't notice it immediately.