Considering how recent pronouncements of the pope have stirred up some cross-faith discontent, it's extremely unlikely in the near future, but IF the UN were to attempt any such thing, I would imagine that it'd be far easier to institute some sort of watered down ecumenism than it would be to ban religion outright. Of course, that assumes somebody could find a way to deal with the radical religous elements such as the Islamic terrorists and such. Something along the lines of the Orange Catholic religion depicted in Frank Herbert's "Dune".
Inevitably, if THAT actually happened, every dub from here to hell and back would be screeching about how it was the prophecy fulfilled, the beast is here, blah blah blah. Speaking from a purely secular point of view, it might not be a bad thing if such a thing happened, and indeed might be a required step on the way to what I believe to be the best case scenario for mankind: universal atheism. What better way to make religion obsolete than to make it universal, and minimally invasive or important?