Not a modder yet (looking for info to work on my own mod now) but I'd think that if you remove all the block damage in the various "weapons" (items.xml) that the zombies use that would prevent them from doing block damage and thus digging. But they'd still do the action. I'd imagine that to stop them doing any digging task you'd have to mod the actual AI which I think is outside what you can do in a modlet.
Is it possible to edit the .dll to make this work with later versions of the game? If so what was changed to achieve this? I've poked the .dll a bit but am not sure what I need to change and all the various tutorials I can find are (it seems) for the purposes enabling Creative/Debug to cheat/hack MP.... I just want to easily remove soil/rock volumes while testing things. Or is there another method to achieve the same end in more recent versions of the game?