I think this question should be in the "Performing Arts" section, because you'll find a lot more experts in that forum section than here. I'm a commercial photographer and usually try to cut out as much ambient as possible so every light in the shot is created by me. But I'll answer it for you real quick.
You want to throw diffusers on the flash head. I use a sto-fen diffuser that fits over the head. If you point it directly at the subject and you have your power set to 1/128, it'll cut the light hitting the subject by about a stop, so it'll be equivalent to 1/256 power. If you point the flash head up into the air, it cuts it by about 2 stops, so you'll be hitting the subject with a light equivalent of 1/512 power if your flash head is set to 1/128 power output. And if you point the flash head behind you but at a 45° angle, you're looking at even less power output. Just aim your flash head in different directions to throw less or more light onto your subject with the sto-fen on your flash. This advice is assuming that there is no ceiling in the venue you're photographing.
But, I'd still ask this same question in the Performing Arts forum section, because there will be guys there that deal with this problem every week.