How do I prevent Insider Program from re-installing Buggy Update?

I had the same issue as you folks did just yesterday. All of a sudden, build 17074 was installed on my machine. The start menu and my apps disappeared, the OS was sluggish, it took me forever to do anything - just as you have been experiencing. It took awhile but I finally got into update and rolled back to a previous version. I then was able to turn off future preview builds and now things have stabilized. To fully disable updates, you have to leave the Windows Insider Program (and if you choose to do this we'll hate to see you go). Not sure if this will help but I thought I'd share.