First, disable the Finder button extension (only macOS 10.14 and older): Go to BetterZip > Preferences > General and uncheck the option "Show BetterZip button in Finder windows". After that quit BetterZip and move BetterZip.app to the trash. If you chose to add BetterZip services to the Services menu (including the Finder's contextual menu), you may want to remove these from the folder Library/Services in your home folder. Lastly, open the folder Library/Preferences in your home folder and remove the file com.macitbetter.betterzip.plist.

https://macitbetter.com/help/