I called a fight once, but against a player. I know that sounds weird, but...
The game was Shadow Run. Someone was selling Sim chips (virtual entertainment) with hyperviolent scenes that were causing people to act them out IRL.
PCs traced it to one particular dealer, so they tracked him, learning the "circuit" of places he dealt from.
They decided to meet him at an Orc biker bar. For some reason the female Elf "Face" character (very pretty, high charisma, good social skills) decided to go in alone. Instead of asking about the hyper violent type of sim-chip the player had her ask about BDSM type porn. She pressed for details, as in, how does one play the sub instead of the dominant,
I called it right there. The player had done every wrong thing possible, and I didn't want to play out the details of what followed. The chances of there not being a fight were zero. The chances of her winning that fight, when that heavily outnumbered (and her not being a heavy combat type) were zero. And the exactnature of the assault that would follow were best left unsaid, if you know what I mean.
Even the players at the table agreed on that one.
Oddly, the Player involved wanted to play it out. To my surprise she managed to fight her way to the door and escape. (Okay, I kind of let it happen, a bit.) Then the general fracas moved to the parking lot, where the other PCs could get involved. Serious injuries ensued, as the Orcs were, how would you say, "highly motivated".
Other than that I generally let the PCs enjoy the romp when they have to face an obviously weaker opponent. Aside from the feel-good they get, it also gives them a chance to burn resources that they might want later, when the big boys show up on the scene.