Open it up, go to Customize > Applications > User > Installer.app - click delete. It'll ask for your Phone IP address and root password, so I think you need to make sure your phone is on the same wifi network as your computer, check on the phone what the phones IP address is (Settings > Wi-Fi > Click the blue arrow next to your Network) and the root password is ;dottie".
This worked for me, I'm not 100% sure how jailbreaking works so might not work for you!