I think you're all wrong!
What the detector is seeing is a specific band of emissions in the infrared.
Tin foil wouldn't stop that, as the foil would be heated by the body and so would register on the detector. Ditto space suits and wetsuits.
The counter measure would be something that emitted absolutely nothing in the spectrum the detector is looking at IE something as cold as the surrounding environment. All you'd need was an absolutely perfect insulator, with the body inside kept warm, but no warmth emitted. So very thick, very efficient artic-type gear would (possibly) work. In fact, on the police videos, you do sometimes see people almost 'disappear', with only their faces showing. I guess they are wearing thick coats.
The face and breath would be a tricky problem to solve though.