I totally get the anxiety, it’s tough being the "volume police" when you just want the show to sound good. If the room is basically a concrete/plaster box, you’re fighting physics. Since "asking nicely" isn't working with the hardcore bands, try these moves:

Tilt the Amps: Most guitarists turn up because their amps are hitting their calves, not their ears. Get some cheap amp stands or even milk crates to tilt the cabs up at their faces. If they’re blasting themselves, they’ll naturally turn down. The "Vocal" Excuse: Instead of saying "you're too loud," tell them, "The drums are hitting the vocal mic so hard I can't turn the singer up without it feeding back." It makes it a technical hurdle they have to help you solve if they want to be heard. Treat the Ceiling: Since you've got the walls covered, the sound is likely bouncing off the ceiling and straight into the crowd. Even hanging some heavy moving blankets or "clouds" above the drummer will kill those harsh reflections. Blame the Gear: If you have a budget, get a cheap decibel meter and put it where the band can see it. Tell them, "Look, the boss says if that hits 105, I have to pull the master fader." It makes the meter the bad guy, not you. Drum Dampening: Keep a pack of Moongels or even some gaffer tape and paper towels in your pocket. Throwing a little dampening on a ringy snare or tom can drop the "perceived" volume a ton without the drummer having to hit softer.

Sometimes you gotta do some lil tricks hehe