Is the Site Doubler ("may be held in a Site that's already occupied by another Lost Soul") an instant ability or on-going?  I'm thinking on-going but not sure.  I ask because of targeting purposes, such as Three Woes, which would kick out Site Doubler if it's on-going.
If I have Assyria's Tribute active ("While blocking with an Assyrian, negate heroes...), and I block with an Assyrian, and my opponent has Ham ("Flood survivors have site access") and Noah's Wife ("Special abilities on your blue Gen. 6-10 cards CBP") in the field, would I be able to negate Ham's site access ability on flood survivors?  I'm assuming I can just interrupt it but not prevent it?