This is the instruction from ClamXav site
22. How do I uninstall ClamXav completely?First thing to do is quit ClamXav Sentry (if you use it) and make sure it's not set to launch at log in. You will then need to find the ClamXav disk image which you downloaded when you installed ClamXav (or download the current one again) as you will find an uninstaller application on that disk.
The uninstaller will remove the engine and any schedules you've got set up. All that's left is to drag ClamXav.app to the trash.