Yup, just keep hitting it with weedkiller. be patient, don't try and get it out until it has been dead and withered for some time (weeks) or you will leave small pieces of live root in the soil and the while nightmare begins again.
Trouble is its in my flower bed which is really pretty at the moment, so at present I,ll have to keep pulling it up.Normally I just get on with it, but I've got raging toothache so I,m fed up.
then the solution is to lift the plants you want to keep, pot them up or plant them elsewhere and trash the whole border. when you re plant use a weed membrane.