Can anyone tell me the name of this weed and how I can get rid of it? This grows in my neighbor's yard every year. It is starting to invade my yard and blocks the sun from my butterfly bushes. A different neighbor had it cut down last year, but it is back with a vengeance this year.

It gets pretty tall (some stalks are already taller than me and I'm 5'3"). The stems are thick and hollow. I live in Southwestern PA. I've also noticed this growing along a stream nearby if that helps. Someone once told me it was polk, but it doesn't have berries and doesn't quite look the same to me.