How to block bluetooth signals?

Yes, enjoy the biscuits first, but make sure they come in a metal container rather than a plastic one.

If you have, say, a Bluetooth dongle on a USB extension lead, just cut a hole large enough for the cable to pass through the metal tin and put on the lid. You could block around the cable hole with metal foil too, but the overall signal attenuation will still be very high without this.

Conversely, you may be able to just place your device in the metal tin.

Even simpler, wrapping the transmitter (or receiving device) in metal foil will work - maybe try a nickel-coated antistatic bag:

Edit:

A colleague has just suggested putting the device on a 'variable distance transportation system':

Try, if you can, with a Class 3 Bluetooth dongle. Here's one for example. It should have range of about 1 meter (I haven't tested that one, it's just the first Google found).

To use a metalic can as an RF shield, you must first ground the can through a thick short wire ground to the RF source, and then also solder the can lid at several places to provide and effective signal shield.

Place it next to a microwave oven and make popcorn. (Most microwave ovens emit very strong 2.4GHz noise, which is the same frequency used by Bluetooth and Wi-Fi) You might need to combine this with the Faraday cage (tin foil etc.).