This is something of a vexed question.
There's a sliding scale of divas: at one extreme, we have the people who flip out because they are tangential to the production. They aren't all that important or significant, and flipping out is a way of manifesting that urge to feel "wanted". Nobody's paying enough attention to me, nobody cares about my input, I don't know why I'm wasting my time with these losers, so I'm going to piss all over the floor because at least that way people will remember I exist.
At the other extreme, we have the professional divas who flip out because they know they have us by the nads (the author, the star upon whose name the entire production depends, the director, the producer's wife, etc.) and they simply enjoy making the Little People turn funny colours.
Different types of divas require different responses--but it's also important to consider that different directors require different responses.
I'm sure we've all worked with directors who wouldn't want us to intervene even if the actors were literally stabbing each other to death. ("I don't want to take sides! Let's not rock the boat! Just let it play itself out! This is an ordinary part of the creative process!")
And at the other extreme are the directors who throw up their elbows and come out swinging the very second someone looks like they might act out. (Or perhaps they prefer the velvet-glove-iron-fist approach: they won't say anything in the room, but they'll quietly visit the producer after rehearsal and have the offender bounced from the company by tomorrow morning. That'll show the others...)
In an ideal world, here's how I'd hope to address it:
(All of this assumes it's a non-equity show, and that the diva fit is of the conventional TANTRUM TANTRUM TANTRUM FLOUNCE OUT OF THE ROOM style.)
First Offence: take ten, go calm the person down, talk about their fee-fees, fawn over their ego a little (if that's what they need) and bring everything back together as if we've just had a smoke break. We all have bad days, not a big deal.
Document the incident.
Second Offence: if at all possible, the company keeps rehearsing without them. They can take their time coming back, and I won't report the absence unless a higher-up absolutely insists. If they aren't back within 30-45 minutes (enough time for a good proper cry in a toilet, if that's their thing), place a call and find out where they are, but don't go running after them.
Schedule a meeting between the person in question and the producer to discuss their grievances.
Document the incident.
Third Offence: keep rehearsing as above, but this time refuse to re-admit them to the rehearsal hall until they've met with the producer. (They'll be paid for the day, but they aren't welcome in the space if this is how they're going to behave.) This can't keep happening, and if it does, I will recommend the actor be discharged--assuming the producer doesn't do so of their own volition. Final warning, pull your socks up or go back to waiting tables.
Document the incident.
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As I said above, this approach is only workable in an ideal world. You can't always bounce an actor (seriously, you're bouncing your star after you're already in previews? seriously?), your director and producer won't always back you up, the tantrum won't always take that idealized form (fleeing the room and weeping in the coat closet is quite different from staying in the room and hurling invective at your coworkers), etc.
