They may move on once they run out of food on the island, but I wouldn’t rely on that happening soon. Beavers will travel a fair distance from the water for suitable trees, and 75 yards isn’t much of a barrier for them. If they still have access to the trees across the road, they may simply start using those instead. If they aren’t causing problems with your dam or shoreline, I’d keep an eye on them for now and see whether their activity decreases once the island is stripped of the remaining trees. A trail camera would be useful for figuring out whether they’re still regularly coming back.