You either need a crossover unit or simply a low pass filter. If you put your low pass filter right before your sub power amps you can control which freq. are allowed through to the subs.
If you want the free way to help with your problem,
use one of your Auxiliary outs to send to the subwoofer. This way you can only send certain things to the subs and leave others all the way down or out.
For example: if your have your subs hooked into Aux 6 then you can turn up aux 6 on your bass guitar channel, kick drum, and maybe a little on your keys. This will engage the subs for those specific channels only.
Be careful though, because this does not replace what a low pass filter/crossover does. You are still going to get unwanted frequencies i your sub, but at least you are limiting them which should clean up your mix.
Neighborhood Church, Redding CA