DJM 900 Nexus digital output

I assume the Utility attenuation settings won't give additional headroom from clipping the DJM.  Historically Pioneer's attenuation pads have been either in the analog domain or after the audio data becomes fixed-point.  The levels that are represented by the master meter feeds both the SPDIF and master out.  So if you pad it down, you're just padding the earlier brick wall.  The attenuator settings on their mixers are usually just to match the DJM outputs to outboard gear's input requirements.  If the sound guy's done his job setting those attenuators, downstream gear shouldn't distort further even if the DJM is as hot as the DJ can run it.  Doesn't mean the DJM itself isn't distorting, though.  As for SPDIF inputs, using them prevents you from distorting the DJM input, but I digress.

Put the master volume knob at 2:15 o'clock so its meter is the same level as the channel meter when the fader is wide open, stick a cap or blue Neutrik barrel over the master volume, and put a label on the top of the DJM that says "stay out of the red".  If you see people intentionally & frequently into the first red 10dB of accident-only last-resort safety headroom, reach over and turn their gain/trims down sufficiently and warn them.  If you see them doing that continuously after a warning or running into the double red ("clip" on the NXS2), then turn the sound system down by at least 75% (to one quarter of) its rated power to protect it from damage or kick them off.

Oh, and turn the limiter off.  Limiters on digital gear get abused.  You don't want people crushing into one intentionally.  Limiting and clipping both increase RMS and are risks to the sound system.  You don't want to prevent one obvious problem by introducing a less obvious problem as a solution that is also going to endanger the sound system.  Leave the limiter for the studio environment when recording and streaming.

Despite that not everyone will aggree, I still think that a proper quality limiter in between the mixer and amplifiers it the best option. Locked away to prevent abuse.

I'm actually talking about the limiter on the DJM-900 which should left for studio, not live venue, use. The DJM-900 already has an unpleasant-sounding brick wall limit with it turned off due to its digital nature.  That brick wall limit is 0dBFS in fixed-point.  It cannot exceed the peak output represented by the double red master meter LED ("clip" on the NXS2). Putting another limiter between the DJM's master analog output and the amplifiers at a higher threshold than the DJM's set max possible output controlled by the attenuator settings in Utility is a potential safety measure if the DJM should malfunction or have a cable or power problem. Not likely to be much of a risk even with a malfunction, but not completely impossible. I wouldn't bother with it or want an extraneous piece of gear to degrade the sound between the DJM and the amps, but do whatever floats your boat.  If this outboard limiter threshold is lower than the set max possible output of the DJM, however, then the limiter is only going to add further risk to the sound system, not less.  If you're using the SPDIF, an additional peak limiter between the DJM and the amps has zero benefit.

The DJM-900's limiter function replaces harsh-sounding clipping with pleasant-sounding look-ahead peak limiting. This is mostly bad for a live venue environment with a large, expensive sound system. Only the cheapest of completely unprotected modern pro tweeters are at risk of damage from the DJM’s clipped signals' ultrasonic harmonics... assuming they even made it that far.  If you're at risk of killing your tweeters that way, you either need different tweeters or more of them.  The main risk to quality pro high frequency drivers used as intended is from clipping amplifiers when the amps output multiples of their rated power into the ultrasonic range, which is why nearly all pro amps now have their own built-in limiters. Clipped source signals, however, do not present additional risks to any woofers outside of the audio signal’s compression as you crush into the brick wall -- compression that look-ahead peak limiting also causes. So if you turn the DJM's limiter on in a live venue, you're incentivizing the compression of the audio signal and risking voice coil damage, not preventing it.  Doing stupid things on the DJM should sound bad in a venue so the DJs stop doing it.

Reticuli, after using the DJM-900 Serato (NXS) for quite a while I never knew that you could turn off an internal Limiter... I'm using the DJM-900 Serato mainly for studio mixing, so I'm curious how to set this limiter function on/off.

Is this done via the Club Utilities Menu?

In this Club Utilities menu I've already set the Master Output to the lowest dB possible, combined withe the lowest input sensivity of my Genelec's. This results in a full 0dB Master VU meter output, and workable sound level.

Yes, through the Club Utilities menu.