You can usually delete a package via the repos web interface. If you click on the package filename or "details" in ss3, "Delete Version" and "Delete Package" will appear in the Actions on the left.
n.b. unless you have a compelling reason (e.g. accidentally committed personal info) it is generally considered bad form to remove versions that have been released to the public, as it can break users' builds. If you move the repo, for example, one option is to commit a set of empty package versions that contain a marker indicating the repo has been moved. This way, users depending on specific versions will still have access, and users wanting the latest will get the notification.
Cheers,
Sean