How do I COMPLETELY uninstall Avast anti-virus (free edition)on my 2007 iMac usingTerminal?

Back up, first. Before making any further changes, try to preserve your data.

Failing the Avast removal tool discussed in the replies above, expect to wipe the Mac, and reinstall, and migrate documents and settings and not apps. How to reinstall macOS - Apple Support with a link to erasing included there.

Anti-malware can be difficult to distinguish from malware, with how it ties into the platform.

And some of the anti-malware removal tools can fail.

And removed anti-malware apps can leave latent corruptions, which removal tools and other forms of app cleaners can miss.

For those unfamiliar with some of the history of this particular vendor: https://www.vice.com/en/article/qjdkq7/avast-antivirus-sells-user-browsing-data-investigation

You are wise to get rid of Avast. It should have an uninstaller on their web site. Avast is very difficult to install all of its various bits and pieces. I would use their uninstaller. It should do the complete job. If one is not available, the app I have successfully used for years to do removals is AppCleaner. It's free. Open it and drag the Avast icon into the presented window. It will then offer associated files for you to accept by selecting them. Clock the remove button and it's done.

A more reliable method is to check with the Developers of this Software.

It is their software and it is their responsibility to Support their Software