Even when I do NOT start CCleaner v6.21 (on 64bit Win10) I can see that CCleaner writes something once a day to the file "BugReport.log" in subfolder LOG.
Why?
Is there a background service?
I can't see it.
How can I forever stop this background service and writing to all *.log files?
The bug reporter runs daily to see if there are any error logs from CCleaner.
If there are error logs then it will send them in for analysis and close itself. If no logs then it will do nothing and close itself.
It isn't running all the time as a background process, just for seconds once a day.
TBH I wouldn'd and don't bother about it.
If you really want to then:
You can disable or delete the daily task in Windows Task Scheduler. You could even delete the bug report executable so that the Windows Scheduler has nothing to run. (Of course doing that will cause Windows to write an error to it's own error logs when Scheduler can't find the .exe).
However they will simply both be recreated next time you update CCleaner.