As said after screw is out, hit the CENTRE of the disc, as I have found this to be a LOT more effective than clouting around the outside, they don't get very tight on sierra's compared with say a mondeo.
On some cars I have even had to saw through the disc splitting it in two, as this releives a lot of tension making it a lot easier, good luck.
I always use copper grease between hub disc to prevent a repeat performance, after thoroughly cleaning the hub face.
tabetha