Acanthus Mollis

I bought an 'Acanthus Mollis a number of years ago and now I want to get rid of it but it has taken over the small square patch that it is in. I have tried all ways to get it out of the ground but it is a bit like a Dandelion, if you leave behind a small bit of the root up it comes again.
I have also tried weedkiller on it but because the leaves are big and shiny it doesn't work.
The flowers on it when they appear are quite something but now I want a change and put in some annuals or biennials where it is standing. I thought about getting some tree killer but is that going to extreme?

 

Any thing with shiney leaves, i use my big feet and crush the leaves as much as pos then give it a spray of weedkiller