What is the best advice you have to get rid of thistle in my beds?
Be persistent. Spraying with Roundup (Kleenup / Killzall) will eventually kill them, but do remember they have an extensive root system. So every time you spray, you will kill the plant you see and some of the roots. If it pops back up, kill it again with some more of the roots. Persistence, and eventually you should get it under control. Nuke ‘em, nuke ‘em, nuke ‘em, and nuke ‘em. It may take all season.