Uninstalling Avast Security from Mac | Avastsupport.avast.com
You can check to see if you've removed all of the supporting files by downloading and running Find Any File to search for any files with the application's name and the developer's name in the file name. For Avast software you'd do the following search(es):
1 - Name contains avast
Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.
FAF can search areas that Spotlight can't like invisible folders, system folders and packages.