do you have a picture? some people would love to come take that bamboo off your hands. just place an ad on craigslist. if you just want to kill it naturally, try cutting it back to the ground and then break or step on every new shoot that pops back up. depending on how much energy was stored in the rhizomes, it may take another growing season to end the attack.

Chemicals are unlikely to work. What part of WA are you in? Some of us might be willing to dig it out, if it's an interesting species. Can you post a picture here?

Here is a link that might be useful: Controlling bamboo

If you like it, just prune where you don't want it- if your able. Cut unwanted areas to the ground. Not that you haven't thought of that.... but maybe it doesn't have to be senseless work. Depending on your space, time and the species, perhaps you could dry and then sell the canes. A bit of a payback for your time.

Is it bamboo? Or is it the dreaded Japanese knotweed? Some pictues would help, before we all jump into mistaken conclusions.