Negation

One of the better bits of advice in the 4e DMG is "say yes". It's not an absolute rule, of course, and in particular "yes" stands on a continuum ("yes, and", "yes", "yes, but", "no, but", "no", and "no, and what's more"), but the principle is sound - and it is that, as much as possible, the DM shouldn't negate the player's preferred choice of action. So if something might work out, even if it's not what the DM envisaged, it's better to allow the attempt than to simply rule it out.

Which is cool. A player's character is the one thing they get to control in the game. The DM gets to control everything else. That being the case... hands off!

But there's another part to that: it applies to the other players, too. If one of the players at the table declares an action, even if it's clearly a stupid action,it's not appropriate for another player to jump in with "I stop him from doing that." Especially if the second player's character could only know the action was stupid after it was done.

(For example, if the big dumb barbarian decides to call the king a cowardly little shit in front of his court, then the player of the bard shouldn't declare he stops him from doing so - by the time the bard realises just how bad it's going to get, it's already too late.)

Of course, in addition to messing with the other player's agency, the other issue there is that you're also messing with their portrayal of their character. If Bob is playing a big dumb barbarian, it's entirely possible that the character is exactly the sort who would insult a king to his face, and damn the consequences. And, yeah, that's going to make it a damn sight harder to 'win' the game, because the group have just made a powerful enemy, but it actually doesn't hurt the game if things go wrong in interesting ways.

But, really, it's the 'agency' thing that's important. You have your character to play; leave Bob to play his.

And, of course, it's important to note that this is also not an absolute rule. If Bob's barbarian is heard to utter "I can jump that far" when he manifestly can't, the other characters might well urgently persuade him not to do so! But, in general, it's better to let the other guy act and then deal with the consequences, rather than step on another player's toes.