Besides stopping the service? Well, you could try an authenticating firewall and only whitelist the service account used for the QRS service. Disallowing access to https://machine_name:4242/* from machines other than itself might work.
"Stopping the service" meaning Qlik Sense entirely?
No, just stopping the QRS service. Yes, that actually does mean effectively shutting down Qlik Sense entirely. It's not a serious option. The QRS API is access at port 4242. If you can prevent access on that port to any connections not originating on the machine itself or from other machines you control (like rim nodes if you have a multi node setup), then that might do the trick.