If you must cull them here are a few ways...
You can enclose them in a back and put them in the freezer. Or you can put them in a bag and shoot co2 into the bag. Either way I'd say is rather painless. Another way is tosse a rubbing acohol soaked paper towel in a bag full of them. I would say tho the best way to get rid of overstock roaches is to give them to a pet sotre or sell them. Or someone who breeds lizards in your area. If you want to slow down thier breeding I'd lower their temperature by about 5-10F. That would slow down the breeding alot. Luckily I don't really have an overstock problem becuase of my monitor lizard and tarantula collection. The monitors are eating machines especially, they can eat a dozen roaches a day each if I'd let them. When you thin the roach population down, handpick the runty mature roaches and the ones with bad colors if possible.