I'd chop them down as far as poss then carefully apply some Rootout, if you can get it - it's set to be withdrawn from sale this year under EU regs.
On that issue, I must say as an application to finish off tree stumps and stubborn brambles and when applied carefully, sparingly and most of all - sensibly, I've never had anything but good results and nothing else in the surounding area has been harmed or affected in any way.
Oops! Got carried away there.