A club owner friend of mine asked me, "I can't seem to get my DJ's to keep their music down to safe levels. What is the best way to protect my sound system?"
You can have a sound engineer and/or a DBX Driverack 260 limiting the master signal but one thing that remains the same is your DJ will ALWAYS crank it up. Remember, the DJ is NOT a sound guy, he is a performer; and whether it's good for him or not, he will experience ear fatigue through the night and he will continue to "crank it" to maintain his perception of "being in the zone".
The sound guy can keep taming his volume down while the DJ fights it until his DJ mixer goes into distortion, or he could crank it at a strong compression level and the music will just sound weird, but the best investment you can do, so the DJ doesn't have to crank it, is to get a big monitor system.
LET HIM BLOW HIS EARS AND NOT YOUR SOUND SYSTEM.
If he's in the zone, you can tame your master sound and he won't even know. In a piratical sense, his monitors have to be able to out-power the mains and its echo back to the DJ booth, so one monitor is not enough. Two maybe sufficient, but I found that at Union Bar in Iowa City, four is actually used effectively, with the added option of two subwoofers.
It can be a costly investment to get a good monitor setup, but remember, it's cheaper to replace or repair a few $200 monitors than it is to replace or repair a $10,000 sound system.
I personally don't crank my monitors too loud, just enough to drown out the mains so I can hear my scratching. But there are times the monitors are just not enough, which makes a DJ tend to crank the master. Club owners, if you want to keep this from happening, give him enough sound for himself. Telling him repeatedly to turn it down is futile, because what sounds loud at 11pm can be extremely quiet at 1am. Not to mention the years of abuse his ears have been, his perception of loud is not the same as yours, especially from the isolated position of the DJ booth.