How to prevent access via reflection?

In Java docs it mentioned that using f.setAccessible(true) method we can violate the principal of encapsulation.

But if I am writing any class which has full security, for instance with a private variable, how can I prevent it from being accessed with reflection?

For example I have a class with full secured instance variable:

public final class Immutable {
    private final int someVal;

    public Immutable(int someVal) {
        this.someVal = someVal;
    }

    public int getVal() {
        return someVal;
    }
}

But I can modify that instance variable using reflection like this:

public class Tester {
    public static void main(String[] args)
            throws NoSuchFieldException, SecurityException,
            IllegalArgumentException, IllegalAccessException {

        Immutable i = new Immutable(10);
        System.out.println(i.getVal()); // output 10
        Field f = i.getClass().getDeclaredField("someVal");
        f.setAccessible(true);
        f.set(i, 11);
        System.out.println(i.getVal()); // output is 11 which implies some value modified
    }
}

In my code, how can I prevent an immutable class being changed with reflection?