If you want to achieve that, you'll have to disable CyberCapture, Reputation services and CommunityIQ. All can be found in main settings.
Though, reputation service by itself is only a query system which uses the cloud backend to check for file status. It doesn't actually send files anywhere, it only sends its hash. CommunityIQ however does that as well as more in depth file characteristics, but it doesn't send the whole file to avast! as far as I know, just file type, structure and metadata. Only CyberCapture sends the whole file to the cloud VM scanner.