Does anyone know about lockout devices for doorknobs?

I've seen before these hard plastic or metal boxes that can be locked on top of a doorknob so you can't grab onto it. It makes it impossible to turn the knob until the box is unlocked. If there's a keyhole in the knob, it covers it up so you can't unlock the door either. I think they've been used before to lock tennants out when they don't pay their rent (although I think that use is illegal now). Does anyone know what these are called or where to get them?