To my knowledge, there is, in fact, no strict limit on the authority the UN can have - but there may be some specific carve outs to other international bodies (i.e. laws of the sea - the division of antartica), however I think these carve outs were mostly made affirmatively by the UN to empower other organizations.
There is a separate question on whether the UN has the power to enforce legislation it passes, but that's a pretty simplistic reduction that can be applied to pretty much any governmental body and will almost always result in the answer of "It depends".
There is no strict limit to the authority I can have. That doesn’t mean I can prohibit Google from doing squat. I could theoretically get the U.S. government to pass a law, but that’s not the same thing as authority.
At a basic philosophical level laws and police have no power, nothing can stop you from committing murder. The only thing we can essentially do is give you pain in reaction - that pain could be pre-emptively acting guilt or fear of the consequences, it could be moral regret, it could also be physical restraint or torture. Given diseases like alzheimer's and essential tremors or a plethora of other physical and neurological ailments the list of people who can absolutely control your actions might be an empty set[1].
That all said, the UN has been granted some pretty wide authority to pass treaties that are generally acknowledged across the world - those treaties are generally respected and the US might hit you with a big stick if you violate them. So, honestly, I think the comparison in authority to an individual is pretty thin.
1. I think that by definition if we're talking in absolutes then it is always an empty set because there are actions you can't voluntarily execute, like, for instance, willing your heart to stop. But let's just assume that "absolute control" is a level of control achievable for most people at some point in their lifetime.
That's the difference. You can go on technicalities, but being technically correctt is the worst kind of correct.