[My friends can't do anything when their un op]

Hey CoolProgrammer sorry idk how to correctly reply. The only plugin i have is ViaVersion! (plz tell me how to correctly reply i cant figure it out for the life of me)

[My friends can't do anything when their un op]

Hello. Recently I have run into a problem me and my friends started a world and they can't do anything other than walk when not op. We want this world to have no cheating and its quite easy to cheat with op. I have tried changing the spawn protection to 0 but it still doesn't work. I have also made sure their in survival. It started after someone random joined the world (because I forgot to private it) and at this point im about to give up. I wanna know if theres a way to fix this.