If they are the common ones, then they are hard to kill, but they shouldn't come back if you remove all the rosettes.
Never watering them and never feeding them is a good way to keep them alive here. I have a whiskey barrel full of them that's been ignored for over 25 years.
You can try watering them well, especially before a good freeze. That should rot them. If that doesn't work, then wet them down good during a good hot day and that should rot them out.
If you weren't so far away, I'd come and dig them up. I've got a good place for them.
