what pin? you mean the key?
that key can as well stay in place.. just completely remove (to be sure they're loose) the two grub screws..
(you can thread them back in by hand a turn or two to remember where you put them later on

)
but make sure they're loose.. if you think the key has coldwelded itself to the axle and disc, then yes... whack it until it moves (either way is fine)
if you can remove it completely.. that'd be nice (it'd give you the chance of rotating the disc while keeping the axle fixed...
that could rattle some rust and paint loose as well
if you can't; get a box cutter and scratch away some of the rust and paint along the seam...
it also help to induce some heat and force
(i.e. take the propane torrch and carefully heat the hub of the disc)
as soon as that's certainly a few degrees hotter than the axle itself get a piece of scrap wood hold it against the hub face right on the axle, and give it a whack with a hammer...
not too violent (you don't want to slip and bend anything

)
then some more penetrating oil and back to start...
once you can move the disc ever so slightly..
you can take some sanding paper and clean the axle in front of the disc to help it slide off.
'sid