firstly as far as i'm aware, the only way an account can be banned is if a transaction doesn't complete properly.
the way this happens is if, you have a friends password and log on to his account on your PS3 and purchase something, the money will not pass from your friends card/bank account account to Sony, whilst his account can still download the item from the store and you can install it on your ps3 for use with other accounts. in this instance his account will be permanently banned, until the owed money is payed by him.
what often happens, is someone will get a password for an account, then login and spend money without the account holders consent, they will install the game or content they downloaded and use it on their ps3 with their own accounts. after this the person who had stuff purchased on his account without giving consent will owe sony all that money.
you may think this is fraud, however its down to the person whose account it is to keep their password secure and not give it out.
therefore you cannot be banned for obtaining the password to someones account and using it, unless your retarded enough to admit you deliberately committed fraud, the sensible thing to do is to say you had been told the password by the friend and been given permission to spend money via a conversation over headset
happy defrauding suckers guys