Witch is simply a System Preferences panel witha background task that catches your kestrokes (in order to display the switcher window). To uninstall Witch, first disable it (remove the checkmark next to Enable Witch on the Witch System Preferences panel). This stops the background task. Next, return to the top-level of System Preferences, control-click on Witch, and select Remove "Witch" Preference Pane from the contextual menu (it's the only entry).

Quit System Preferences, and you're done - no need to logout/login or restart.

If you want to get rid of Witch's static files—its settings, etc.—delete these two items:

1. The Witch folder in your user's Library → Application Support folder.
2. The com.manytricks.Witch.plist file in your user's Library → Preferences folder.

Keep in mind that if you install Witch again in the future, you'll have to create all your settings again.