Eliminating Cook Top/ Oven Combo Box

I don't think usually the pieces are practically severable. You'd have to do something to alter the gas manifold to eliminate the oven thermostatic valve (or at least plug the outputs of that valve), at the very least. As with many things, given enough time and/or money and fiddling it could undoubtedly be done and done safely, but getting a proper ovenless cooktop is likely to be easier and possibly more economical.