Read this: (I cant explain) https://www.spigotmc.org/resources/fairplayminecraft-client-side-anticheat.17071/ How to solve with the hackers... (Im not done with the buying plugin..)
Just asking people to download an app and run it won't make people do it as half the players don't even check forums and many don't understand english. Even servers like badlion can't afford to lose players this way so they kept the normal server and made a client side anti cheat too. This would just result in a huge loss of players.
Install this app is optional. But when you installed it and checked, you have some advantage (Something that normal player didn't have and it isn't op), also able to /votekick non-checked member. This encourage normal play to install it. If you a hacker, not installed it. well, you can get kicked ass many times when you hacking and get votekicked You dont need forum to install this app. Just login to gc server and in the lobby it have the link to the app. YOu can list the trusted mods to the plugin... This is just temporary solution.. not so long...