I'm with Ex as well WL. I hate province poaching as both the attacker (in most cases apart from as a rusher) and the defender (in most cases and especially as rush defender).
As the attacker (but not as a rusher, as rushing has special conditions) it means someone is basically saying to me "Oh hi there, you've done all the hard work in killing that guy, so do you mind awfully if I help myself to a few provs by joining the war at the 11th hour and denying you of some spoils?". That's like an action movie where the Cavalry arrives just as all the fighting ends and the movie is fading out to the closing credits. With in this case the Cavalry expecting to take a share of the glory.
So if I was in clean-up mode in a RAND game, and someone suddenly poached a few provinces, especially if they were well out of their zone, then I'd likely react with potential hostilitiy as well, and do exactly as CY did. Capture the poached provinces /only/ and see if the poacher wanted to make anything of it. As if I wanted to start a war, and had the capability to launch a huge first strike, then /only/ going for the poached provinces should send a pretty clear message of my intentions IMO. As if I had really wanted a war then I certainly wouldn't have started it in such an underwhelming fashion, and for that reason my actions should be fairly clear in that I was only (re)claiming provinces that I felt I had a very strong right to, or very strong need for (eg. empire split in half without them. Or precious resource, such as Astral mage for non-Astral nation)
If the poaching was just of some stray provs well inside the poachers borders, meaning it was just a border straightening excercise, then I might turn a blinder eye. But otherwise no. Far too many players are opportunists when it comes to pinching others prey, and innocently think there's no harm in grabbing a tiny slice of the pie for themselves. Never the case in my experience.
And as defender I hate it even more, especially in rushes. In two RAND games I was fending off a powerful nation when poachers turned up to make my life harder (impossible) by squeezing my income and cutting off my counter attack options, which did nothing except make the attackers life easier. The theory is that you're depriving the rusher of extra provinces should they win, but I'm fully convinced now, after being the unsuccessful rush defender twice, that this is just BS.
As the rusher is likely happy to have a few less provs to reduce the effect of the leaders tag his rush will have, and so reduce the chance of a ganging. (I know I've been happy to see poachers when I've rushed). Plus as the rusher a few less provs means less time needed to clean-up and move-on, and less provs for the defender means less resistance and so easier to take down. And whenever I see this happen, a few less provs from the rusher has never once weakened the rusher, as if they've had the capability to rush someone, then when they want more provinces they simply declare war on their next target and take them. They couldn't care less if their rush netted them only 10 of the original 12 provs their target had. The capital is all a rusher really cares about, and making sure they take it asap so that they don't get bogged down in a prolonged first war. And often the rushers next target is the poacher themselves who conveniently moved their borders closer to the rushers force sitting in the newly aquired capital, and doing so allows them to reclaim the poached provicnes as well. Two birds with one stone, how convenient
Too much poaching gets done as an automatic responce to seeing a neighbour going down. Once again showing Dom players are generally poor at making strategic decisions, and often act well in advance of doing any actual thinking (RTS syndrome perhaps). The only thinking they tend to do is after the game in hindsight "yeah looking back now I shouldn't have done that obviously stupid thing. I really should have thought more about it at the time, but I just thought what the heck".