I assume you're referring to Japanese knotweed.
I got rid of it (a small area) by digging up as much as I could and covering the ground with plastic weed suppressor which stops the light getting through. Any bits that showed up later (or the following year) I treated with systemic weedkiller. I did this 2 years ago and it seems to have worked.
My mother had this plant growing up through a lawn, so I could not employ that solution there. The best thing to do here is to cut the stalks down in August when they're at their biggest - but before they flower. It's important to cut at this time because you stop the plant repleneshing its food supply for the following year. This will not kill the weed but will weaken it and if you repeat the process for a few years, the plant will eventually die. If the plants are small, you might get away with just using systemic weedkiller. Sorry but there's no instant solution. Do not discard ANY fragments of the plant anywhere - it will start to grow again from a small piece of root, etc. Let it dry out and then burn it. There are several articles on the internet which discuss this weed.