How shield bluetooth radio to stay in one room

I have the rather unusual wish to *diminish* the reach of a bluetooth sender! I want the signals to stay in one room, because I want to use its detection as an automatic indication for a smart phone to know that it is inside a specific room. I haven't tried with a bluetooth class 3 device yet, because I can't find any for sale! But class 2 devices easily send signals through even two of my walls, and they are solid concrete walls since I live on the 12th floor, so that it covers my entire flat from a corner of it.

What characteristics does bluetooth (2,45 GHz) radio waves have when they hit different materials like concrete, metal or water (as in wood and humans)? Are they absorbed, reflected or pass right through? I would guess that they are absorbed by fluid water molecules and reflected by metal, and from my little testing, they seem to care little about concrete. But there are maybe tables describing how different radio frequencies behave in different materials?

How could I with simple means either:
1) reduce the signal strength, or
2) direct the radio waves?

Maybe it'll be enough to reduce the strength by wrapping the sender in something. Maybe steel wool?
Maybe the signals could be reflected ("paraboled") by small aluminium plates towards the outer wall, which I don't mind if they pass through, because then at least they do not enter the wrong room.

What would your advice be?

The best is to modify the room as a Faraday cage with aluminum foil covering of all walls, windows and floor. In a particular case with real transmit and receive devices, if the transmitter and receiver are provided with e.g. SMA connectors, then you can enclose the devices in metal enclosures, to only allow the RF signal to pass through the connectors.
Then you can try to connect the devices by good coax cables, and add a variable attenuator to try how much loss you can add to propagation loss to still operate the radio link.
By using an adjustable attenuator to/from antenna to a transceiver, you could limit the range possibly within a room. But- RF can also be conducted along power and phone lines, radiated through walls and windows, water pipes, etc.
So try to attenuate to find a limiting distance. Then try again outside of the room if a signal can get through. Propagation of RF signal in a free space is predictable, indoors, rather not so.

Yes, covering the walls would be nice, but not feasible. If I understand you correctly, a secondary ideal way would be to completely isolate the transmitter except for an "antenna" which could be chosen and adjusted at will until the range is optimal. Luckily, my experimentation with improvised low cost household metals seems promising!

Putting my S7 class II bluetooth "hands free" in a sauce pan with its cover on, reduces its range to about 1? meter. Which is a way longer range than I had expected, but I suppose sauce pans are made of stainless steel and therefor not very conductive.

Covering the bluetooth hands free completely with one layer of aluminium foil, drops the range to about 50 cm (half a meter)! I think it is very promising that it did not block radio contact completely. The foil I tried with is marketed as "extra strong". Maybe there are thinner foils around. Or else maybe making some small holes in it could increase the range to room level. Two layers kill all contact at any distance.

I've also tried with steel wool (wire wool). When folded into an ordinary "sheet" of it, no contact was possible. Its flashing LED was still visable through it. But by stripping off layers, contact was achieved inside the same room but not in the neighboring room. This seems to be ad hoc scalable to any range from case to case. But dirty and a bit time consuming with trial and error. It seems to be very important that the cover is homogeneous. One open hole (say 1 cm diameter) in any direction relative to the receiver, easily makes the signal come through even to the next room. Somehow, bluetooth signals seem to manage to both penetrate through, AND reflect from, concrete walls!

I will have a look in shops what kinds of malleable metal grids I can find to experiment with. Maybe one could wind a metal wire around it?

Concerning the conductivity difference between steel or copper, in this application it is not important. Metals block "your" radiation equally well. Now, I am glad that you succeeded with wrapping your transmitter (or receiver), to attenuate the signal and reduce communication range. Please be aware that the signal often travels not like a light beam; there may be other beams, reflected ones, etc. All depends on location of your transmitter and receiver, and how tightly wrapped they are. You can find that a weak signal is received in one situation while much stronger signal can go another way, also through room walls. To be sure, I would suggest to use several receivers in various locations.