How do I fully close the Wiser Radiator Thermostat so that I can drain down the radiator to take it off the wall without draining the whole system?

You should not use the Wiser Radiator Thermostat to close the valve when removing a radiator.

When removing a radiator ALWAYS completely uninstall the Wiser Radiator Thermostat and then close the valve manually using a decorator cap. 

Once you have reconnected the radiator please re-attach the Wiser Radiator Thermostat and calibrate the device. 

First, please ensure that the radiator thermostat is firmly attached to the valve.
Twist the cap in the '+' direction for 8 seconds.
Release the cap and allow a moment until the device displays a solid green LED with a red & blue LEDs flashing either side.
Then please twist the cap in the '-' direction for 2 seconds.
The LEDs will go off and the device is now calibrated.