A datapack that looks like this doesn't remove two handed trait from the warhammer:

{
"values": [
"spartanweaponry:armour_piercing"
]
}

A datapack that looks like this doesn't remove the hammer slam but does add the new traits to battle hammer:

{
"values": [
"spartanweaponry:extra_knockback",
"spartanweaponry:nauseous_blow",
"spartanweaponry:armour_piercing",
"spartanweaponry:two_handed_2"
]
}

Am using the latest version.

Pretty sure it's because ModWeaponTraitTagsProvider just flatly adds the traits without consulting with existing data json files? So you can't actually remove any of the pre-set traits via datapacks.