You could cremate it while it is in the graveyard. The REALLY hard part is to find a way to get it into the graveyard.

Text from Legacy Weapon
Text(AP): {W}{U}{B}{R}{G}: Remove target permanent from the game. ; If ~this~ would be put into a graveyard from anywhere, reveal ~this~ and shuffle it into its owner's library instead.

The word "instead" indicates a replacement effect. This means that if you try to put it into the graveyard, Legacy Weapon will require you to shuffle it back into the owner's librard INSTEAD of going to the graveyard.

That makes it extremely difficult to put it into the graveyard. About the only method I can think of involves animating it and turning it into a creature, then using Humble or Humility to remove the ability, and then killing/destroying it.