the signal output from your mixer board is controlled by the master stereo fader. the sound engineer just have to make sure that his LEDs are not always in the red. This is controlled by the sound engineer. (That's why we need him).
between mixing board and the spearkers, there's another piece of very important equipment - the amplifier. This is a very expensive equipment and blowing this cost more money to fix then the speakers. (we talk about standard equipmet setup here ok).
we can insert a compressor/limiter between the output of the mixing board and the input of the amp. this way, if the singer accidently swing a microphone into a bass guitar. the transient outbust will activate the compressor/limiter to kick in and pad this loud sound from being proportionally amplified by the amplifier.
answer of the day : look at something along the line of compressor/limiter.
worms_eat_birds, is this a problem you are facing? in what situation do you face this problem?