Main mod file:

@Mod(modid="examplemod",name="Example Mod",version="v1.0.0")
public class ExampleMod {

    public static ForgeEventHandler eventHandler = new ForgeEventHandler();

    @EventHandler
    public void postInit(FMLPostInitializationEvent event){
        MinecraftForge.EVENT_BUS.register(ExampleMod.eventHandler);
    }

}

Event handler:

public class ForgeEventHandler {

    @SubscribeEvent
    public void onEntityMounted(EntityMountEvent event){
        //Is a EntityPlayer mounting another entity?
        if(event.isMounting() && event.entityMounting instanceof EntityPlayer && event.entityBeingMounted != null){

            //Is the player is holding brick?
            if(((EntityPlayer) event.entityMounting).getCurrentEquippedItem() != null && ((EntityPlayer) event.entityMounting).getCurrentEquippedItem().getItem() == Blocks.brick_block){
                //Don't mount the entity, and send the player a error message.
                event.setCanceled(true);
                ((EntityPlayer) event.entityMounting).addChatMessage(new ChatComponentTranslation(("You can't mount this entity while holding brick, it's too heavy!", new Object[0])));
                return;
            }

            //Send the player a message saying that they have mounted an entity.
            ((EntityPlayer) event.entityMounting).addChatMessage(new ChatComponentTranslation((event.entityMounting.getName() + " mounted a " + event.entityBeingMounted.getName(), new Object[0])));
        //Is a EntityPlayer dismounting another entity?
        }else if(event.isDismounting() && event.entityMounting instanceof EntityPlayer && event.entityBeingMounted != null){
            //Send the player a message saying that they have dismounted an entity.
            ((EntityPlayer) event.entityMounting).addChatMessage(new ChatComponentTranslation((event.entityMounting.getName() + " dismounted a " + event.entityBeingMounted.getName(), new Object[0])));
        }
    }
}