Put workbenches around your base to prevent spawning inside your moat. I usually carry a small beam blueprint around so it will show me the craft radius. That way I can make sure there are no spots that aren't covered. After I have the whole area covered, I replace the workbenches that I don't want with campfires. Campfires will do the same thing, but they're less likely to get broken when the mobs throw things. They don't have to be lit to work.

As for the current raid, you have to be inside the red circle for two minutes to get it to end. Run around as much as you want, but stay inside the circle. Since the trolls are inside the moat, you can sprint jump across (if it's only one square wide) or down one side and up the other. The trolls can't follow you across it, assuming there is still space inside the red circle for you to be on the outside. Then you can finish them off with a bow when the raid ends. That's what the moat is for after all.