Dealing with the War on Fungus Gnats, Part 2.

Even after spraying the entire basement with proper insecticides, filling the only water source with ammonia, and keeping lights off the majority of the day, the gnats returned shortly after.

There are no plants down here. 1 water source that is cleaned. Usually dark, and no rotting wood/fungus that I have ever seen. Why do they persist? Could they be in the wall space?! Is there a last resort method of extermination short of calling an exterminator to fumigate? They're driving me insane.