There are many reasons people can be banned. I am assuming the ones you have seen were banned because they are underage. Legally no one under 13 can be on the board and once we find out we have no choice but to ban them. There are some members who joined before the rules changed and we have asked them not to post unitl they turn 13. But there are some that joined and lied about their ages in order to join. Now that will get you in trouble as it always gets found out somehow.
Other things that can get you banned:-
Repeated bad language
Being rude to members
There is a list of rules that everyone should read, it is not long and it is very helpful. It is on the left hand side of the screen under HELP/INFO.