since the crankshaft will turn as you try to loosen the nut, you need to use the inertia (at rest) of the crankshaft and pistons. how? e di gamit ka ng "impact force" - use a hammer on the wrench. lagay mo yun wrench sa nut then hammer it in the direction it needs to turn (note that the nut may be a left-hand threaded nut, as oppose to conventional right-hand thread).
or you can try removing the starter and putting a wedge on the flywheel teeth to prevent the crankshaft from turning.
the 4 bolts kilawind is talking about is that sometimes there are 4 (or sometimes 3) small bolts on the pulley in addition to the nut on the shaft. i'm not familiar with your engine (or even my car's engine hehehe!) so i can't really tell if there are those 4 bolts.
you may also need a small puller to remove the pulley from the shaft.
tama si sir yebo puwede kang gumamit nang martilyo para mag loose yong nut, ang down side naman neto is baka ma damage mo yong tread ng crank shaft sa sobrang force pero subukan mo parin ingat lang sa pag pukpok baka daliri ang tamaan.