Sorry for not being preciser in the first post:
When I opened the config.txt for the first time, I found that the “dtoverlay=pi3-disable-bt” was already there. My aim is to turn off the wifi too, so I added a “dtoverlay=pi3-disable-wifi” and rebooted.
I was surprised to see a list of available wifi networks in /Setup/Wifi.
In the meantime I found that the wifi is actually turned off, i.e. not listed when running command ifconfig.
Nevertheless, the available wifi networks are still listed. Perhaps this is just a cosmetic flaw and it remembers previously scanned networks?