Question: Can be the rights assured by the Constitution in a democracy be revoked if I happen to sign a document which explicitly states so?
This questioner specifically asks about a reality tv show contract.
This assumes we are in the United States. It depends on to whom you are giving those rights and how you are giving them away. You would not be signing away your rights under the Constitution when it comes to a tv show, because a tv channel or tv producer is not obligated to your rights under the constitution. The constitution protects you from government actions, not the actions of a private company.
There are some laws that prevent you from signing away certain rights. I am thinking for example of certain rights that tenants have in various states that specifically may not be signed away. But there would have to be specific laws that prevent the signing away of those rights in the jurisdiction in question.
If a contract has issues and you decide to sue over it (or are sued over it) there is always the chance that the court could find the content of the contract to be against public policy and that could offer a solution, but I certainly wouldn’t rely on it. For example, a court won’t enforce a contract two people enter into for illegal activity. So if you purchase marijuana in Pennsylvania, even if you have a signed contract that involves the deal, no court is going to enforce it if one side decides to re-neg on the deal.
So, if you agree to sign up, for example, for a reality show, and you feel your privacy has been violated, there isn’t too much you could do about it if the contract protects the behavior of the tv show’s producers.
That doesn’t mean you couldn’t sue, and possibly even win. It just means that, as you suggest, your best bet is to avoid signing the agreement in the first place.