Disable the WP Admin Bar in 3.1+ entirely, or only for roles and capabilities which aren't in the 'whitelist' or 'blacklist'.
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.
If you're interested in showing the Admin Bar in more cases, check out the Always Show Admin Bar plugin: http://wordpress.org/plugins/always-show-admin-bar/
If you're interested in showing the Admin Bar to logged out users, check out the Logged Out Admin Bar plugin: http://wordpress.org/plugins/logged-out-admin-bar/