CancelAllTheThings.java

final AbstractEventRegistrar aer = new AbstractEventRegistrar(this);
aer.registerAbstractListener(InventoryEvent.class, new Listener() {
    @EventHandler
    public <T extends InventoryEvent> void onInventoryEvent(T e) {
        if (e instanceof Cancellable) {
            final Cancellable c = (Cancellable) e;
            c.setCancelled(true);
        }
    }
}, EventPriority.HIGHEST);