I'm assuming you want to fix the fact that people can use linked attack in siege? Instead of disabling the use of party skills. Instead disable the damage modifier of linked attack while people are in siege.
Simply go to attackarbiter.cpp and search for:
Code:
nDamage += pAttacker->CalcLinkAttackDamage( nDamage );
and replace it with
Code:
if(pAttacker->GetWorld()->GetID() != WI_WORLD_GUILDWAR )
{
nDamage += pAttacker->CalcLinkAttackDamage( nDamage );
}
In case your server has multiple siege/pvp worlds then don't just check for WI_WORLD_GUILDWAR but also check for the other worlds where applicable.