I don't know what yours looks like specifically. But here are three
suggestions:

1.  Make a holding tool such as the one shown here:
http://www.miata.net/garage/LongNoseRepair/index.html

2.  Put the transmission in gear so the pulley will not move. Use a
breaker bar and socket. Position the breaker bar so you have to apply
force in the upward direction to loosen the nut. Put a mechanical or
hydraulic jack under the breaker bar and jack up the bar until the nut
breaks free.

3.  There are those who use a variation of (2) by resting the end of the
breaker bar on the floor and engaging the starter. Although they swear
by it, I have my doubts as to how safe and sane it is. Also, you now
have to re-align your crankshaft and cams. This isn't too bad for those
who know what they are doing...but why bother when you have two other
options.

Good luck,

Ken