All thistles have long tap roots that must be dug out or the plant will regrow from that. However, after repeated cutting of new top growth that root will eventualy die from lack of food. Thistles are also spread by the birds, mostly finches, that love the seeds the readily produce. The only real means of control is to just keep cutting them down.

Thank you so much for the info. I will definitely try that since it's probably easier than getting the roots out anyway. I'll be out there today cutting off the tops of every plant I can find! It's a relief just having another method to try. Donna