Note: if you only get this kick on a specific server then chances are a mischievous plugin developer is faking the message to prank/troll you. This is not allowed and the plugin can be reported, but you don't need to worry: it isn't possible for plugins to globally ban you.
Otherwise this means your Steam account is "game banned" from Unturned. Steam game bans are applied by BattlEye anti-cheat, so if you think this was incorrect please refer to BattlEye support here: https://www.battleye.com/support/