Drew showed me this nice trick to zero out a Plaxo account (something I need to do a lot because of testing).
Simply go to Manage Folders, Create a new Contacts folder, and then delete the old one.
Warning: I don’t know what happens when you sync down to your desktop client. My solution is to run the Plaxo for Mac Uninstaller (in /Applications/Plaxo), zeroing my account, and then reinstalling Plaxo. This will have my desktop data overwrite Plaxo.