How to switch off smart cleaning

"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:

https://www.ccleaner.com/docs/ccleaner/ccleaner-settings/changing-monitoring-settings-ccleaner-professional/system-monitoring