the one that comes to mind is a proscription against entering battle under false colours, which in the Traveller setting would equate to using false transponder signals.
Note that "entering battle under false colors" really means, "At least 1 second before we shoot, we raise our real colors."
Sailing along under false colors in order to entice the enemy nearer was, AFAICT, a universally-recognized-as-acceptable
ruse de guerre.
Pirates, in many cases, would be more likely to raise their real colors earlier than actual military ships, since pirates' goals were much more focused on taking and plundering rather than sending potentially lucrative prizes to the bottom. Moreover, it was "common" practice to steal heavily from a ship that surrendered without fighting but send them on their way (to hopefully steal from them again), and to offer no quarter to any ship that fought - a bit of nautical psychological warfare. An early surrender, then, was far preferable to a drawn-out fight.
A great book to pick up is [ame=http://www.amazon.com/Under-Black-Flag-Romance-Reality/dp/081297722X/ref=sr_1_1?ie=UTF8&qid=1302115870&sr=8-1]Under the Black Flag[/ame] or the even-more-period [ame=http://www.amazon.com/General-History-Pyrates-Daniel-Defoe/dp/0486404889/ref=pd_sim_b_2]General History of Pirates[/ame].
(The second is listed as being written by Daniel Defoe, but the last time I read it the actual authorship was still being debated - that Defoe was actually the author Captain Johnson was proposed but not proven. The version I own carries Johnson's name, rather than Defoe's.)
If you haven't read it and want some more space pirate inspiration, the 6th X-Wing book -
Iron Fist - follows Wraith Squadron as they go undercover as a pirate band.