Code 710 was a communications code, instructing ships to under no circumstance approach the planet sending it.
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 interplanetary war they were fighting with Vendikar, another planet in their system. (TOS: "A Taste of Armageddon")
In Star Trek IV: The Voyage Home, the Federation President issued a planetary distress call advising anyone who might be listening to "avoid the Planet Earth at all costs", a directive very similar to Code 710.