After I bought a new Apple TimeCapsule and set it up as my wireless router/base station, I noticed a constant 0.3KB/s down, 0.1KB/s up network usage, even when I wasn't actively downloading/uploading anything. (I use iStat Menus 3 which shows the network activity in the menu bar). It started to drive me crazy.
I fired up Wireshark and started to monitor the packets, and I found that my computer and the base station were constantly communicating on port 192 (Wireshark displays it as port osu-nms). According to this Apple Support Article, port 192 is used for 'AirPort Base Station PPP status or discovery (certain configurations), AirPort Admin Utility, AirPort Express Assistant.' It appeared that my system was constantly checking with the base station, about 4 or 5 times a second.
After searching the internet for quite a while, I finally found an answer on a Japanese blog in a post titled AirMac-UDP-192 (page is in Japanese), in which the author was having problems with SystemUIServer and during trouble shooting, started to remove all of the menu bar icons. It was at this point that they noticed that the traffic on port 192 suddenly stopped when they turned off the AirPort (AirMac in Japanese) icon.
And it works! Turning off the AirPort menu bar icon in System Preferences » Network » AirPort stops all of the port 192 traffic.
I've been running this way for a month or two with no problems; AirPort Admin Utility still works fine, so does Time Machine. And now when I'm not uploading/downloading anything, I'm really not uploading or downloading anything.
[crarko adds: I tested this, and it works as described. It's obviously an inconvenience if you need to move between a number of access points and need to select them manually, but if there are security or other concerns this is a way to turn off the scan without using a firewall to block the port.]

