How can I disable automatic scanning for WiFi networks?
AFAIK there is no way to disable wireless network scanning. You can only disable the alerts about new wifi networks being found.

More significantly, because of the way wi-fi works, the transmission rate is going to be very very low due to all the stations interfering with each other.

But reading into your question at a deeper level, "How can I get decent wifi"


Google "pringle can antenna" You will find a raft of recipes for high gain directional wifi antennas.  This can be used to select a given station out of the crowd.
This, by itself leaves you leashed to an ethernet cable to the external antenna, which you really don't want to move.
So you connect the pringles cantenna to your own wifi router, set it to broadcast on a seperate channel (ideally >3 channels away from the one the cantenna is listening to)


Finally, you may find that the easier way to get anything done is to use a wired conection.