The Ban Hammer is a special weapon that resembles a gravity hammer from Halo. It is, predictably, an incredibly powerful weapon... In the challenge.
It's possible to use the weapon with the Fiesta modifier; However, the weapon will not have its trademark ridiculous damage output, instead being an average melee weapon. It will still cause ragdolls to go flying upon a kill, however.
Mike uses this as a melee weapon in Bigger and Badder.