Been a few years so if it is also a bad thing to do I'll plead stupidity. I had a couple of gallons of bad gas to get rid of so after having it around for awhile I used a heavy open top can that held probably 4 gallons I dumped in about 2 gallons of bad gas and set it out in an open space away from everything and pitched a burning piece of paper in it and let it burn. Burned for a long time and removed all the paint from the can but it got rid of all the gas. In the case of the OP I'm not sure what you do with the water that is left but it may just evaporate.