How evil would it be to enforce direct subclasses only?

public abstract class AbstractType {
    protected AbstractType() {
        if (getClass().getSuperclass() != AbstractType.class) {
            throw new AbuseOfInheritanceError();
        }
    }
}

I've seen so much code that was made less readable and more complicated by overuse of inheritance; there are cases where it would be doing the world a favor.

However, probably it would just make people want to shoot me.