Disable building

Hey, I would like to disable build on my spawn island, but not during the game. The players spawn first after joining the empty map, on the spawn island. There they surely cant built. But after the game strart they spawn first on the spawn island again, and then they get teleportet on the map. The problem is they can build after the game start on the spawn island. How can I disable the build mode on the spawn island?
Thank you <3

Drop in a Mutator Zone and make it big enough to cover the spawn island, it has an Allow Building setting

Thank you so much <3