Disable the WP Admin Bar in 3.1+ entirely, or only for roles and capabilities which aren't in the 'whitelist' or 'blacklist'. Get rid of the Admin Bar when you don't need it, and keep it where you do!
Pretty easy setup.. just install and activate it on the site of your choice (or network activate), and setup the settings however you'd like.
This plugin supports both Per-Site and Per-Network (WordPress Multisite) settings, so you can fine tune your options based on your needs.