I'm not certain how much of the Iberian Peninsula you hold yourself, but there is a strictly limited number of land routes through the Pyrenees. One army in the pass along the road blocks it, one army in the pass with no road blocks it. It might take two armies or even three to block the two coastal routes, but they can be blocked. If the other crusaders aren't allied with you, then you can tie them up almost indefinitely with zones of control. Of course, nothing stops them from going by sea, but they don't seem to do that the majority of the time.

You may also try the assassin route, assuming you have a good assassin or two. Kill off the generals of the crusading armies and they'll eventually go away. I've never seen an AI army suffer from desertion on a crusade or a jihad, even if it doesn't move for decades, but they certainly do disband if they lose all the generals and can't get a new one in time.