I have stopped my Item from breaking, but I want the tool to stop being useful once it has hit it's max damage.  The tool would act as if it's just an item.. i.e. a stick.

I think if you return -1 from getHarvestLevel, the tool will not be able to harvest anything. If you also return 0 (or some low value) from getDigSpeed it will behave pretty much like a stick.

These two methods recieve an ItemStack as parameter, so can check if your tool is already broken and act accordingly.