Ideally I would want to upload my updates (only) to a test server and my test client would check for updates there, not on the production server.
Right now there's no easy way to release to a test server, test your updates, then roll it out to a production server. We're aware this is a problem and we're adding it in the form of "update channels" that will be added in the near future.
In the meantime use separate wyBuild projects for your "test updates" and your "production updates".
Also: Is there a way to revoke updates or clear out very old updates that I know nobody will need any more?
Delete them from your servers. We don't do this automatically as part of the uploading process for a number of reasons. Plus, harddrive space is so cheap it won't make a difference for 99% of our customers to leave old files on the server. But, like I said, just delete the old updates from your server. They're no longer referenced from your *.wys file, so they're safe to delete.