How can delete the serve.bannersdontwork virus from Safari on Mac?

This is not the work of malware. At most, it is being caused by some adware that you were fooled into installing, but it could very well be something else as well. See:

http://www.thesafemac.com/eliminating-browser-redirects-and-advertisements/

From the Safari menu bar, select

Safari ▹ Preferences ▹ Extensions

Turn all extensions OFF and test. If the problem is resolved, turn extensions back ON and then disable them one or a few at a time until you find the culprit.

If you wish, you may be able to salvage the errant extension by uninstalling and reinstalling it. Its settings will revert to their defaults. If the extension still causes a problem, remove it permanently or refer to its developer for support.