If you have access to the previous account, delete the thermostat from the account by logging into the Honeywell My Total Connect Comfort account and selecting MY LOCATIONS listed towards the top of the screen.
Select the cogwheel labeled SETTINGS and scroll down to locate the Mac ID and CRC number for the thermostat.
Select the TRASHCAN listed next to the Mac ID and Mac CRC numbers.
If unable to access the previous account, please contact Honeywell Support at (800) 468-1502, open from 9am to 5pm CT, Mon-Fri.
For Honeywell Home app users, if you have access to the account the thermostat was registered to, login to the app, select the thermostat, then select the settings cog wheel icon in the top right corner. Select thermostat configuration, then select Delete Thermostat.