It would be interesting to be able to cancel the product addition. But I think it is not very intuitive to do it because we would have to return success => false in hook_add_to_cart and as ca_pull_trigger does not return any value... Maybe an option would be to add it in the node as I saw in another feature request ? Anyway, I finally did it directly in hook_add_to_cart. I also tried to do it with uc_cart_remove_item directly in predicate actions but I found that the product is added after that action so it does not have any effect.

Thanks