Bonide Weed Beater Ultra can killer thistle.  Wait until nigh time temperatures are 50 degrees or above (usually mid-May to mid-June).  Spray the leave of the thistles so they are thoroughly wet…the weed killer is absorbed through the leaves.  Apply when rain is not expected for at leas 48 hours..the longer the weed killer is in contact with the leaves the better it will work.  Kill them before they produce seeds.