I was following runs in a channel and eventually I got a message saying, "You are banned from that channel"

All other accounts I have are not allowed to go to that channel too so I suspect it is an ip ban. What is the reason for this and how to fix?

It would seem the people running that channel banned your 'one' particular account. Leaving your others able to join. Only way you can rejoin would be to message them and talk to them about it.

Someone clearly didn't wanted to in the channel anymore. There's nothing you can do about it then ask the admins of the channel to unban you.

The thing is, I tried going into that channel with another account. A random one they've never seen. Still it says I am banned from that channel. They couldn't have banned it.

They probably run a 3rd party program (channel bot) -- some of which are known to be able to ip ban people. I won't elaborate further on this as it's forbidden to talk about.

Ah, that makes sense. Thanks man.

Report them to blizzard (for using a 3rd party program)

Clan channel can ban by IP to. They also have other option to ban a specific account

channel bots are legal under blizzards agreement.

i know i run 7.

It's not legal. By using it you broke some law. Also some of these 'Channel bot' are used for some bad thing like flood.