The command is bugged and isn't working. Try not to invite anyone that you don't really trust. The only way they can leave the coop is if they leave, you can not coop kick them since the command is bugged. The command might be fixed soon, I hope.
You can kick players from your coop if they are online. Unfortunately there is no other solution until the issue is resolved with the plugin
I will archive this as the responses should be useful. If you need any more help, feel free to open a new thread