There are some jurisdictions that protect software that's running in production. In the case of industrial control software, there can be serious safety considerations to shutting down software. I know this from a previous job with a vendor that constantly had licensing disputes with industrial customers.
I may not be remembering this correctly, but I think Canada is one such jurisdiction.
Just because they have no right to use the software, doesn't mean you can act maliciously. If they have no right to use the work, just ask for an injunction -- a legal kill switch.
