Well that is pretty hard and not terribly good anyway.

I 'accidentally' got caught trying to assassinate a Polish diplomat. War!
Damn... I'm already at war with all my other neighbours (as HRE), I didn't need a war with the powerful Poles. So I stayed defensive, didn't attack them in the hopes that they would calm down.
They made a single foray against me, and I 'gently' pushed them back. Then they seem to have gotten warned by the Pope.

40 turns later, the Polish have refused all my attempts at peace... And now they have begun their first real invasion of my lands. So we had a 'cold war' for 80 years, yet no chance of getting a peace even when I offered to pay them 1500 florins for 10 turns and give them Stettin. That was a very favourable peace for them, and I hadn't even given them any trouble.

Given that I have managed to kill the Danes and Milan, it is about time to kick the Poles in the nuts.