Hmm. I am using a CombinedDomainXYPlot and the methods you cite work as expected for me.
For your CombinedRangeCategoryPlot, what does isRangeZoomable() and isDomainZoomable() return?
Are you perhaps referring to disabling the mouse wheel? If so, try using isMouseWheelEnabled() and setMouseWheelEnabled()
on the ChartPanel object instead.