I have seen peoples pages and under everything says this user is banned. Can somebody tell me the reasons why or who banns people. I have just wondered this for a while

It could be for many reasons. The reasons are not posted on the profiles because we respect privacy, and rather not start further drama or speculation. 

The main reasons are breaking some of the big rules here: 

http://beastkeeper.com/thread/250

Other instant bans are multi-accounts (satellight accounts are banned, main usually stays open. Goods are not funneled into one account when this happens). 

Art theft is another big no-no. 

Rudeness to other players and staff is another big one. Disrespecting the staff and there decisions could also result in a ban. Posting rude, violent, or other content that is not suitable for a conversation with your mother or younger members of societies are also result in a ban. Flaming (or going after a specific user) or trolling (trying to instigate drama/chaos) is also cause for ban. 

Smaller violations can also add up.  

When  users recieve a warning it is notated on a list that stays there forever. 

After three warnings the user is put in "silence" mode. This means the user cannot post. If a person is being especially beligerant staff can enact a silence at any time. This can be temporary or permanent. 

Banning is not done lightly. Usually a discussion is had. 

Thanks for responding. :)