The problem is the underhand way Piriform hides the option not to install Avast after the first time you install Ccleaner.
Avoid the problem in the first place by opening the Registry Editor (type regedit in the search box), then go to HKEY_CURRENT_USER / Software. You will see a key 'AvastAdSDK'. Right click on this, click Delete then confirm the deletion.
The next time you install a Ccleaner update, you will see a check box (already ticked) to install Avast. Uncheck the box, do the install, and hey presto! no Avast.
As soon as the install completes, go to the Registry Editor and delete that key which Ccleaner has created again, ready for next time.