Was banned on an online forum but my old username keeps appearing on other computers?

Ok, this is reaaaly starting to piss me off!!!!! I used to be part of a forum that i loved but i got banned from it. I logged out and dont go on that site anymore. But i decided to just go on it just to view threads and read stuff. I used my niece's laptop(i have never used it before, it was my first time) and when i was bored i went on that forum, and in the top where it says user name and passwrod, it says that i was already logged on, with my old username and password already there. I thought it was strange. I logged myself out and that was that. A couple weeks later, i got a new computer and just wanted to go on that forum again becoz i like to read stuff and ****, again it appeared that this site automatically logs me in without permission. I can see my old username, and everything, except i am not allowed to post, there is a long sentence in red saying "You are not allowed to post,you are band" something like thast. I am confused why does it say that i am banned. I am using different computers. Is it going to keep saying that when i move to a different house? and obviosuly i cant create another username or i would be banned again, it automcatically bans me if i try to creat new username, i tried tht in the past. What do i do????