Whenever I connect my S2 to the power outlet (not to a computer), the Kies application pops up. Where do I disable this auto-activation?
Go to Settings > Applications > Development > and check USB debugging
After that if you connect your S2 to a PC it will connect in USB mode and you won't get the KIES application popup.
Sounds like there's something funny with either your USB cable, the charger or the socket on your phone that's causing it to detect it as a full PC connection, rather than just a dumb charger.
You should be able to get rid of that prompt for the moment by going into Settings -> Wireless and Network -> USB Settings and selecting the Mass Storage mode rather than the Samsung Kies mode.
However you really ought to try with another charger and cable to see if that fixes the actual problem, pure-charger cables shouldn't be detected as a PC, the signal on the cable is different and the phone should detect that.