Prevent automatic firmware upgrades

When I first had my phone the firmware was Gingerbread 2.3.3. and the wi-fi reception was very good all over the house. After upgrading to 2.3.4 via Kies wi-fi reception became very poor. The phone automatically upgraded via air to 2.3.5 but wi-fi was no better. I now want to have the phone flashed back to 2.3.3 to see if it will improve the wi-fi reception but this will be a waste of time if the phone automatically upgrades to 2.3.6 when it is released. Can anyone tell me how to turn off automatic upgrades.