We have one of those Envirosafe fly traps from Bunnings which degenerates into a disgusting cess pool when full of decomposing flies.
We have never seen the refills at Bunnings although I see that they are listed on their website but my wife saw a tip to refill the traps with Vegemite dissolved in hot water.
It works a treat and avoids the putrid mess created by the original bait.
Also, an article regarding a claimed chemical free method for keeping flies away.