I have kind of wierd problem with my Mikrotik RB951G-2HnD .
I live in a flat on 6th floor with router deep inside the flat , and when I am outside on car parking I can catch my wifi from home without any problems using smartphone.
The problem is I don't want my WiFi signal to be THAT strong , best would be if it wasn't going out of my flat.
Most wierd is that this small router have only small internal antenna and it have better signal than Asus RT-N66U I got earlier and it have 3 antennas ...
No matter what value I tried to enter in settings , the signal is always strong.

WiFi settings ( I removed SSID and Radio Name from picture ) :

You can try lowering tx-power more.
It is funny though, because some people complain about this router, that it doesn't have connector for external antenna and the internal antenna is small and insufficient.

Set your "Frequency Mode" to "regulatory-domain" and raise your "Antenna Gain" until you don't have a usable signal out at the car.

Every 3db you raise the antenna will halve your power output.

Please increase your "Antenna Gain" from "0" to "5" and test see the difference ! If allready the signal is strong increase to "7" etec..

try 5dbm of tx power, its the lowest effective level on that model.

if you want to limit coverage even more try disabling all data-rates leaving only 64qam data-rates enabled

you can also implement acl to enforce minimum tx level of clients to connect.

using this 3 settings you can limit coverage even to a few meters (less than 10) on some environments