Hi Givenup,
Canada Thistle (Cirsium arvense) is considered a pest by many and very difficult to get rid of. You can try to control the spread by continually cutting the plants back at the base rather than trying to pull or dig them out. Thistle has a very deep root system and when you try pulling or digging them up, small pieces break off and remain underground so the plants will re-grow. I've read that thistle does not like fertilized soil so I wonder if fertilization of the area would help kill it in your garden beds? Something like Roundup will kill thistle but Roundup is a harsh chemical and will kill all the other plants in the vicinity as well!
