The purple variety is not a problem, as they don't set seed.
The cream and pink prickly variety is the one that has black berries, which the birds happily scatter round our countryside. They are very shallow rooted and therefore easy to dig out, but you have you keep going back to root out the seedlings which come up in a forest of growth in the spot you just cleared!
Yes, I read all about how hideous it is for you there. It is hard to explain to people here, who think it's the greatest plant since sliced bread (?), how it can really ruin a different type of ecosystem like yours. I believe scientists have proven that the true difference is not what color the flowers are; it's in the DNA. Of course if they don't set seed, it's better!