I don't think we're aloud to talk about ethnic cleansing here. If you want them to emmigrate then make them jobless and unhappy (find out what they like and take it away)
Well, the practical ways of getting rid of the Polish Minority is not about ethnic cleansing, as there are no such option in game. (Unless you count killing rebels/fighting wars as ethnic cleansing)
If you have a full citizenship party, convert the Polish POPs into soldiers, send them to fight a impossible war...
I haven't tried this but this might work, that is, to release a 1 province unciv, remember to first build forts before you release. Now, send some exp. force to that unciv (I think about 5-8 would work) or build some divisions from that land you plan to release. Since you got a bunch of divisions of a certain culture you turned into cannon fodders, land them to fight that unciv. Well, your troops will keep dying and one day you'll be able to get rid of every undesirable cultures. Remember to retreat your cannon-fodder divisions when they are about to be annihilated, it is cheaper to reinforce divisions than to throw in new ones.
Since your foe is merely an unciv, no matter how many divisions you lose your WE would still be low.
When I was playing the US, I got a bit too many Creole POPs in the Carribean and thus hard to assimilate to desirable Yankees, I DOW Japan and send my Creole/Dixie/African Minor/South Andean/North Andean soldiers in that meat grinder. The WE forced me to make peace fast...The Japanese are tough foes in the 1910s.
As an european nation, theoretically you could assimilate the entire world (to your culture) if you use this trick. Given enough time, of course.