They often do that by stopping one-sided fights earlier, but in a recent mixed martial arts fight, one referee had to literally step in and physically restrain a fighter who wouldn't stop despite knocking out his rival.
The incident took place in a Fight UK MMA bout between Nathias Frederick and Ahmad Aswad. In the very first round of the fight, Frederick landed a huge left on Aswad who straightaway hit the mat. It was clear that Frederick had knocked Aswad out and the fight was over, yet he mounted his opponent and continued to punish him.
Referee Leon Roberts intervened, but Frederick still wouldn't let Aswad go. This is when the ref put the unruly fighter in a rear naked chokehold so tight that it completely pacified Frederick.
A moment or two later, when Frederick had calmed down a little, Roberts let him go. But man oh man, as the commentator said, this was the slickest chokehold ever executed by a match official in an MMA bout.