I think this is fairly easy, actually. If you get the sense that an opponent is not having fun (through either direct or indirect communication) and you continue to attempt to "kill their grandmother" or whatever, that is unsportsmanlike AKA being an asshole. Silas is certainly right in that the point-of-no-more-fun is subjective and different for all of us. Some folks may lose one expensive ship or pod and be ready to throw in the towel. Others may happily reship again and again for more combat until their wallets run dry.
As roleplayers, I think it does us all a great service if we act with our opponent's OOC interests in mind when fighting one another IC and, as Silas said, scale the stakes to match the participants. In my example of the WHG-Anshar conflict, we do this very informally. Nothing is pre-arranged. We simply let the other group know that we're going out and where they might find us. For RP corps living close to one another, this may be all you need to get more RP-on-RP combat started.
I disagree that we should accept the only viable goal of a conflict between immortals being the destruction of the opponents' will to fight. While logical, this translates very poorly into the realty of playing EVE. Specifically, the people who (role)play these immortal characters
are mortal, have limited time on their hands, and ultimately play the game for entertainment. Curb stomping them into a failure cascade and pretending it's justified because that's the only way capsuleers would be able to defeat each other is crap. It's perfectly viable within the rules of the EVE sandbox, but it's awfully bad manners.
I didn't say talking nicely to them.
This could mean suborning one of their members to spy for you. It could mean finding out who they work with and putting IC pressure on them. It could mean publicly spreading propoganda (truth or lies) about their organization.
I was actually thinking about a situation in which all attempts to PvP-engage an enemy RP corp came to naught; however, through the use of "text RP", Esna was able to get hard evidence of their dirty dealings and use this to place pressure on those who might otherwise sympathise with his target.
Ah, ok. Misunderstood then. Yea, +1 to all that.