Code 7-10 was a communications code used by the Federation Starfleet. Also known as a "quarantine code," it meant that the recipient was to "under no circumstances approach (the originator of the transmission)." It did not allow for exceptions.
In the mid-23rd century, the civilization on Eminiar VII replied to an initial diplomatic overture with this code because they did not wish the USS Enterprise to become involved in the war they were fighting with Vendikar, another planet in their system. (TOS: "A Taste of Armageddon")