The only way that you'd receive this message is if you had a modification installed that's banned on this network. There's no other way that this could happen.
Certain client modifications communicate through the plugin channel (it works similar to the public chat, except this channel isn't visible). When the server intercepts these messages, you're banned for 5 minutes.
If you want to play on MCGamer, you'll have to play with a profile that doesn't have forbidden modifications. If you're using a hacked client, then you already know why you're being banned. Alternatively, you may have downloaded an entire package of client mods and one of them is what's causing you problems -- even though you might not use that particular mod, having it installed on your active profile isn't allowed.
So what should be your next course of action? Play on a vanilla profile or (like most people in this community) play on a profile with just OptiFine. After that you should have no problems logging on.