ProtoAvatar
Fleet Captain
First - between middle S5 - end S5 the wormhole could have been blockaded by a large Federation armada, weapons platforms, mines, etc, etc.
But that would probably provoke the Dominion into starting the war sooner. Remember, Cardassia joining the Dominion caught the AQ powers completely off-guard. Starfleet was stretched as is, what with the previous Klingon war and Borg attack. Klingons likewise. It took time to reposition the fleets. Their shields still weren't upgraded to fully withstand Dominion weapons.
I see it this way: the second half of season 5 was a race - could the Dominion bring ships through the wormhole faster than Starfleet and the Klingons could ready themselves for full-scale war? By 'Call to Arms' Starfleet saw that it couldn't wait any longer but also finally felt ready enough to provoke the Dominion into war. Only then could they approve a plan such as Sisko's.
The Dominion needed no provocation to start the war.
A blokade around the wormhole, however, would have robbed the Dominion of the capacity to start the war for a few decades. No huge dominion convoys full of supplies coming through the wormhole and jump starting cardassian economy, no resources to build ships en masse.
And about Starfleet being stretched - we saw how effective mines or weapons platforms were - using them to blokade the wormhole would hardly be 'stretched' for anyone who could call himself an important political player.
And yes, fleets too - stopping the Dominion from starting the most bloody war in recent history is worth relocating a few dozen ships from the borders/systems/etc - for anyone who's not an idiot. And if Starfleet didn't have the resources to spare these ships after the recent skirmishes (and they WERE skirmishes - compare them to a true was, as presented), it would be so pathetically weak/undermanned, that klingons/romulans/cardassians/etc, etc would have long ago destroyed the federation - which, of course, did not come to pass.
PS - The Federation/klingons/etc had plenty of time to prepare - since end S2.