Do heron decoys work to keep herons away?

No it would not, if anything it would attract more herons by showing other herons that there is a good food source. If you are trying to stop herons from hanging about near a pond then buy a different decoy being a bird of prey (only £2-£3 from a good garden center). You could use a Buzzard, Sparrow hawk or a kestrel. You could also use a magpie or jay which is general pest to the heron because they steal their eggs.