"The hammer falls" means "this is at an end". The "hammer" falling is that of an auctioneer at the end of an auction or a judge at the end of a trial. The phrase should be used to deliver a dramatic, emphatic finality to the issue at hand.
Usage: "The hammer falls <time (optional)> on <subject, topic, etc.>."
Example: "The hammer falls today on Weebl and Bob. Bob is superior (and Weebl is now much flatter)."
Please omit any puns in actual usage.