Even though it won't help you.
You are wrong in here. ::CanHolster returns TRUE/FALSE, so that you need to not only block the original execution, but also provide a valid result by using SetHamReturn....
Code:
// Ham_Item_CanHolster/Ham_Item_Holster
public fw_Item_CanHolster_Pre(entity)
{
SetHamReturnInteger(_:false);
return HAM_SUPERCEDE;
}