Quote Originally Posted by Ldvs
Let them cross otherwise you get excommunicated...
Just use a full stack of peasants in your province and only the peasants will join, if you put the "valuable" units apart. (for some reason they seem more prone to join ).
Good ideas.

Here are some more ways to screw over a rival faction's crusade:
1) lower the zeal of a province by letting an inquisitor burn it for as long as he wants (oddly enough, for me anyway low valour inqs seem to make more bonfires of your population). Low zeal means almost none of your troops get recruited.
2) lower the zeal of a province by letting one of your own crusades sit in a province for a VERY long time ... when it's depleted send it on a suicide mission to get rid of it. Low zeal means almost none of your troops get recruited.
3) raise the zeal of a province with an inq and bishop combo (to reduce the burnings) and load it up with all your trash troops from your early ventures and crusades. The AI's crusade will soak up all your unwanted troops and if the crusade succeeds the AI faction will break itself trying to pay for it, especially mercs.
4) (from advice someone on these boards gave me) break sea chains so your stretch of ships for trading and transport do not allow a path from the province with crusades going through it and the crusade target. Forces the AI to re-route the crusade.