Hi, we have found that nothing kills em!  you could try round-up from sol d'ebre at about 40 euros for 5 ltrs ,( you water it down,) but we find they treat it as a good drink and come back stronger!  We are now trying bleach, and this seems better so far, but time will tell. Also, we were advised to put some washing up liquid in the round up, as this is to stop the weed killer from just sliding off the leaves.  Good luck!