Coaches know the sideline protocol. Warning them that "one more and the HC is gone" is asking for trouble and a short career.
Agreed that that's not the wording anyone should use directly and hopefully most officials know that. But, a reminder to someone on the coaching staff can be considered a form of preventative officiating. No one needs to remind coaches about the sideline protocol (warning, 5, 15), but I know of a few associations that have procedures whenever an UNS or PF SI is called to inform the head coach or designee directly of the foul and what the penalty entails. Every coach knows a 2nd UNS is a DQ, but I've come across many that didn't know a 2nd PF SI is a DQ on the HC regardless of which nonplayer commits the foul. A simple reminder in the right words can prevent a 2nd occurrence. Marking off 15 should do that already, but I've seen that not correct the issue before. There's a way to go about it in being preventative, but I can see how some may feel it's asking for trouble.