either dig it out with as much root as possible or spray with a weedkiller like roundup. it needs to be a systemic herbicide so it destroys the root too.
welcome to GoY too.

thanks for your speedy reply. I thought it would be this drastic. Every year I dig the stuff out. It's amongst a bed of roses so I guess I shall dig these out too! Best thing is don't plant the stuff in the first place. I know diddly squat about gardening. I can just about manage the lawn!!