"Be Excellent to Each Other" comes straight out the film Bill and Ted's Excellent Adventure and its sequel, whose name I forget ("Bogus Journey" I believe). It is, in fact, the mantra of these films.

In essence, it means treat each other fairly and as you want to be treated.

In this case, I would take it to mean don't hack into somebody's account or do anything dodgy to their computers, etc.

Hope that helps!

Google finds that phrase a lot on the web.
One of the links reminds me of the movie "Bill and Teds excellent adventure" with Keanu Reeves. They used to say things like "be excellent". I think it was the motto of their future state. Since that is a cult film I guess that's where the phrase comes from.
The translation in the German version was close to the original: "Seid exzellent zueinander."