"Run CCleaner when the computer starts" is unchecked and CCleaner still run at startup...
Turn off smart monitoring. The preference you found is whether to run a cleaning when the pc starts up.
Changing monitoring settings (note this feature is now rebranded as Smart Cleaning in CCleaner):
CCleaner Free:
https://www.ccleaner.com/docs/ccleaner/ccleaner-settings/changing-monitoring-settings-ccleaner-free
CCleaner Pro: