In my neck of the woods 2,4-D will knock them down but won't kill them at the root level.  I use the Dow product, Milestone.  $1.00 per three gallons of diluted spray, but I use a hand sprayer on foot and spray individual plants so it goes a long way.  And they do not regrow!  (Sprayed a 3 gallon backpack sprayer full today.  Trying to get them cleaned up before they blossom. If the county highway crew would work on their thistles, could make a sizeable dent in the local population!)  I hate thistles.  And once they get ahead of you it's a job to beat them back down imo.

Milestone does, like most weed killers, kill clover and other legumes, (though most trefoil can handle it).  When I hand spray I mostly just hit the thistles however....