I assume you mean an opposing player. Often with plays near/on the sidelines a player may end up OOB and should he contact a coach in the 'moat', it's sideline interference with the first being a warning.
Guys, we should make the warning official by
and signal #15. Just yelling : "COACH GIT OUTA' THERE" doesn't cut it unless a flag and signal follow. Remember to record it as we do with USC fouls.
Agreed, when the behavior merits a warning, the "warning" should begin with a flag. However, you might consider, "an ounce of prevention is worth a pound of cure (sometimes). Preventable behaviors NOT necessitating an official "warning"; i.e. coaches drifting further than appropriate, towards the sideline (BUT NOT getting in anyone's way ) a gentle suggestion, "Coach, give me the line....thanks" is something you can repeat, as often as YOU considerer necessary, without an official "WARNING", helps set a cooperative tone.
Often one, or even a couple, such suggestions sends a message that hopefully eliminates/erduces (unintentional) bad behavior and avoids the necessity of starting the "Warning sequence". It's entirely
YOUR choice if, or how many such suggestions,
YOU choose to give, BUT if they're ignored and
YOU conclude an "OFFICIAL" (Penalty Cycle) WARNING subsequently becomes necessary, you can always add, "I tried warning you" should whining/complaining begin.