Hey,
This only happens when our anti-cheat plugin recognizes that you're logging onto our servers with a client that isn't allowed. To stop this, you can stop using the client that you're currently using (delete it maybe) and make a completely new one. If you need OptiFine to play minecraft you can download it and it will make a client for you. You shouldn't have a problem connecting to our servers with the OptiFine client.
If you believe that your client is legit and you're not using any illegal mods, then you can make a bug report
HERE. Try to include as much detail as possible and remember to use the template provided.
If you have any more problems with this feel free to message me or any other member of staff on the forums. Have a great day! c:
Thread locked and moved to General Support.