If an id is banned, and members are clueless as to the reasons for the same, it just points out to the fact that the system of banning a member is not clear, transparent and can be improved.
Before banning, it would be preferably if the offensive post is marked and a reason given as to which of the rule was violated. This will set an example of post not allowed to other members. Alternatively, another thread (read only) can be created which can update the latest member banned with reasons for the same.
The suggestion of Raghuveer - of graded levels of warnings is good, an exception of course could be of gross violation which can be invoke permanent ban.