It can be, but its bloody difficult.
Having done some a couple of years ago. Get the rubber bush put up on a mandrel or bit of bar thats the right diameter for how much it is getting stretched. Then lob it in the freezer over night. Get a lathe and grind up a sharp point tool with a lot of relief off the underside. Then in a quick move, bung the mandrel in the lathe, high speed, low feed, turn down.
The only thing I've not had success at doing this with is silicone rubber, which is fooking hard work get get a decent finish with.
Other options do included grinding, or heating up a knife or chisel to really hot/melting temperature and turning it off with that.
