There are two ways to uninstall Broadcast depending on whether or not you want to keep the Broadcast settings and link data.
Keep the data
This option will remove Broadcast from your installation, but keeping all the data – contrary to what the WordPress alert window shows.
Remove the data
This option will completely remove Broadcast and the associated link data table from your install. Links can be restored if you change your mind, but that can take a long time depending on how many posts and blogs your install has.