I don't know about easy, but if I wanted to disable PS/2, I'd probably lift the leg of the little inductor that couples the DCS signal into the board, that would turn it into a conventional locomotive.
Of course, if you're running conventional with no DCS, you don't have to do anything, there is no WD signal. If you're running on a DCS layout, why in the world would you want to run a DCS locomotive in conventional mode?