By creating uninstall-only registry keys and values on the Advanced - Registry project page. When it comes to uninstalling your application, Tarma QuickInstall will first remove all registry values that you specified, and then any empty registry keys.
See Modifying the Registry for more information about creating registry entries for installation or uninstallation; note that for the purpose of removal, the registry values should only have their Uninstall action checked. (If you also check their Install action, Tarma QuickInstall will create the values for you during installation. This may or may not be what you intended.)
Note - If despite the previous, the registry keys or values are not removed, it may be that Tarma QuickInstall runs with insufficient privileges to remove those keys and values. This applies only to Windows NT; on Windows 9x all users have unrestricted access to the registry. Try checking the Require Administrator rights box on the Installation page, if you haven't done so already.