Yes you really do need to try and dig some of them if not all of the out. If you like it but want other plants then leave the clump at the back of the area and try to get the others out. Often I find a saw [sturdy bread knife] is a good way to get these out. Then every time you check or look at the bed remove seedlings.

Thank you Seaburngirl, I really appreciate your help. I'm going to have another go at them this weekend to try and dig them out.

That's good - enjoy! Everyone who replied to you originally agrees that this would be the best thing to do. If you allowed them to set seed you'll have to keep a sharp lookout for some years for seedlings. I'm still finding ht odd one after about years.