In order to prevent a jumpgate from being used by a particular
ship, you have to know its access code, which you can then block. This
is easy once the ship is in B5, because that code is logged automatically.
But unless you know *in advance* which ship, with what code, is going to
be coming through from the other side, you can't block access.

Now, during the Earth/Minbari war, EA jump gates were programmed to
block access to ALL ships *except* EA ships with codes that changed on a
daily basis. Implementing that on B5 takes permission from the Joint
Chiefs, since it is a de facto act of aggression, and takes a lot of time
in ANY event...just as shutting down the gate takes hours and hours, if
not longer.

jms