Thanks for writing. Thistle is a tough one because, as you've probably noticed, it has such an extensive root system. If there are a lot of thistles in your neighborhood, it may be very difficult to competely keep the weed at bay as new seeds will enter your yard every year. But to get rid of the ones you have, the key is persistence --- either spray, dig, or cut back and cover up the plants that are currently growing. Re-attack them each time you see regrowth; if you keep on them, you will eventually exhaust the root systems.
Do you have a layer of mulch over your flower beds? It's not a miracle cure, but it will definitely help to have a 2-inch-deep covering of some sort of material (pine needles, shredded wood, pecan hulls, etc.) over your soil. That will prevent many of the thistle seeds from sprouting and help knock back the root systems that are in the soil.
---Justin, Senior Garden Editor, BHG.com