Your first born has disgraced the family and run off with someone inappropriate. How can you make sure they don’t get any of your fabulous chattels?
Well of course the first step is to write a will leaving them nothing. But in some jurisdictions they could challenge that Will (your Will!), and if a court feels you have acted unjustly they might award some of your hard earned wealth to that dastardly blaggard.
There’s no Will that you could write that would prevent that.
However, if that court knew what you knew they would be sure to agree – or at least be a lot more sympathetic to your views.
So what you could do is write a statement to be kept with your Will in case the will is challenged. You could detail your position and explain why any reasonable person in your position would do exactly the same thing. This could be read to the court in support of your Will.
Alternatively you could include this information in the Will itself.
But that might backfire against you. For a start the person you are trying to exclude, might not want your money and might not challenge the Will in court. But if you explicitly mention them in a negative way, they might be so annoyed that they do decide to fight for a cut. So it would seem sensible to “keep your powder dry” and don’t “show your hand” until it is proven necessary. Which is not a rude euphemism,
But the thing to bear in mind, as I like to remind people, THIS IS NOT YOUR PROBLEM. You’ll be dead and buried when all this is going on.