Here is a recent quote from the wonderful Heisenberg on fungus gnats:
"A great preventative and suppressant for fungus gnats is the bacterium Bacillus thuringiensis. If you use mycogrow or great white you have this bacterium. If you do not use these products, or want to introduce more BT to your tea, pick up some mosquito dunks. Break off a little of the disk and add it to the tea brew, or put some directly into the res. The larvea will still live for several days but stop eating almost immediately, get sick and eventually expire. If you can pour some tea directly on the larvae you have the best chance of infecting them. A turkey baster works pretty well. You should always have several yellow sticky traps around your garden to catch adult females and those who escape the tea. Removal of alternative breeding grounds helps alot. Keep cat boxes clean, trash taken out and left over food put away. Remove any standing water including unused drains. I find these measures are enough to keep fungus gnats from being a problem."
the mycogrow he mentions is MycoGrow Soluble, available from fungi.com for cheap. this can be brewed into a tea and used to inoculate your plants and provide benefits in addition to gnat death. cheers and good luck on your eradication
mr.bond