Unbelievably P###ED off by continued spam.

It all started a year or two ago i got a few smses every now and again from DirecAxis like you usually get from people advertising their goods. i rep**** back with the opt out keyword then it went quiet for a while the they started to phone me and hanging up one day i picked up and someone finaly spoke and said they are from directaxis I told them i opted out and dont want them to contact me but to no avail because shortly afterward they started sending me post to my house phoning me and daily smses i dont know what to do about this **** the national optout website does not respond on any of the multiple devices and computers that ive tried to get my details removed from. anyone able to advise me on how to go further and if i can take legal action as a last resort because this is nothing short of harrasment.