I had this happen on two of my studs when I modified my y-pipe. Basically what we did was heat what is left of the stud up with a MAP gas torch. We then continued applying heat as we pounded the stud out with a sledge hammer. If you have a bench vice, just open it up wide enough to lay the flange on it so that the stud has room to move out. The are in there REALLY well!

2004 Focus PZEV 2.3L - KW sport suspension, SP camber plates, Eibach front swaybar, FS intake, FS Stealth exhaust, 3.82 Final, Torsen ATB, Prothane MM insert, E1 CSVT wheels 1/4 mile - 15.201@89.4mph