What is the correct way to write a udev rule to stop a service under systemd

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http://unix.stackexchange.com – I'm running Arch Linux, and I have a udev rule which starts a service when a device is inserted. In this case, it dials a connection when a 3G modem is plugged in. KERNEL=="ttyUSB*", SYMLINK=="gsmmodem", TAG+="systemd", ENV{SYSTEMD_WANTS}="[email protected]" However, if the device is removed, systemd won't stop the service, and hence when it is plugged in again, it won't start the service, since it's already running. What I need is a matching udev rule which runs when the device is removed to stop the service. (HowTos)