2 Answers

You can trap them in a bottle, take a water bottle, drill some holes in the neck about 1/4 in size.  Put a few (maybe 4) around the bottle on neck, maybe an inch from cap?  Put some wine, or other vinegar in with a little water.  You can also use water and rotted fruit, but the vinegar doesn't smell bad.  I use Balsamic Vinegar, nice and aromatic for them, and tasty!  Fill it up about 1/4 of the way up the bottle, strong solution is best.  Put cap on, and wait.  The little buggers will smell the stuff, fly in and get trapped.  They may live for a few days, and you can take it outside and release them by removal of cap, and then set trap up again. 

Hm...eliminate the source, and perhaps consider refrigerating it next tme.