If enabled, CCleaner’s Smart Cleaning feature checks the junk levels on your PC and alerts you when you have a large amount that can be cleaned. This feature runs as a background process to ensure you can receive these alerts without needing to keep the CCleaner window open.
To stop this, switch off Smart Cleaning by unticking the checkboxes in the Smart Cleaning menu. If Smart Cleaning is disabled, the background process will close and will not run when you next start Windows.
These instructions relate to CCleaner v5.46 and onwards. If you are not on this version, please update CCleaner and follow the instructions above.