Just expel them with /is expel [Player] There is no way to keep them off ur island bc they can just fly to it but u can get them off with the expel command
If you have a warp sign, make sure to remove it, and if anyone have access to your island, simply do /is expel [ign]
I do not have any warp signs since I don't know how to make them. No one should have access to my island except for me. since I haven't added anyone to it.