I just picked up a used CT100 for $15. Thought it was a really good deal, until I found it linked to the previous network. Unfortunately, the recommendations above were not successful in un-linking. I suspect you may still need to have the original controller.
I did find some instructions that worked. It was buried in one of the Radio Thermostat support pages for a different device. See https://radiothermostat.desk.com/customer/en/portal/articles/2024247-zwave-rtzw-02-module-information.
Here’s the buried verbiage that ultimately worked for me:
Factory Default Restore
Performing a Factory Default Restore will return your thermostat to the state
it was in when it left the factory. This means that all of its configuration and
parameters will be returned to their default values. This includes resetting its
Z-Wave network information, removing it from its previous network. This action
should only be used in the event that the network primary controller is missing
or otherwise inoperable. To perform this action, the thermostat must be switch
to OFF mode. Wait until the radio wave icons on the left side of the screen stop
blinking then touch the screen once to turn on the back light and then hold the
center of the screen for about 8 seconds until the thermostat beeps.