you can make a health pickup with the health value set to 0 to eliminate the player instead of a damage volume.
theres a couple ways:
1.) make the spawn limit 1 in the class designer or the team manager for only the spectator class or team. keep the spawn limit on unlimited in the island settings.
2.) make a spectator spawn pad and make a button that switches the player to a spectator class then picks up the health pickup, that will make your player spawn on the different spawn pad. There is a bug in creative 2.0 where they cant press the respawn button if their spawn pad is inside a damage volume, but you can get around that with an explosive device if you have to!
once they become a spectator, there is no way for them to get back unless they go back to the lobby, or a new round starts (and your 1v1 may not have multiple rounds technically in the game files).
1865-9822-2691 theres my boxfight map as a working example, but this one has multiple rounds