What usually must be shown to make out a case of undue influence is:
“a fraudulent influence over the mind and will of another to the extent that the professed action is not freely done but is in truth the act of the one who procures the result.” Id. “‘There are four general elements of undue influence: (1) a person who is subject to influence; (2) an opportunity to exert influence; (3) a disposition to exert influence; and (4) a result indicating undue influence.’” http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=nc&vol=appeals2005/&invol=041266-1
The validity of this sort of clause depends on the law of the particular jurisdiction. For example, Anglo-Canadian common law tends to frown on penalty clauses generally, whether in wills or contracts. The problem with a clause like this is that it’s not restricted to “nuisance suits.” What about the case of an elderly testator who has come under the undue influence of a particular person? or a testator who is suffering from mental illness? The common law generally allows potential beneficiaries to bring a challenge to a will in court. On what basis can a testator immunise his will from the common law of probate?
As well, if a jurisdiction has passed a law authorising courts to hear applications to vary a will, such a clause might be held to be contrary to the public policy set out in the statute, and therefore void.
Here’s one example of a Canadian court refusing to enforce an in terrorem clause: Bellinger v. Fayers, Nuytten et al, 2003 BCSC 563, a decision of the Supreme Court of British Columbia, which held that such a clause was void on public policy grounds, because it conflicted with the policy set out in the B.C. Wills Variation Act. (I don’t know enough about the issue to say if courts in other provinces would reach the same conclusion; would depend in part on the common law, and in part on the particular statutes passed by the other provinces.)
So, the answer to this question depends, as always, on the law of the particular jurisdiction, coupled with the facts of the particular case. You’d have to consult with a lawyer skilled in probate law in the particular jurisdiction for an answer.
diggleblop, I noticed that you’ve not had any replies to your questions, probably for two reasons. The first is that they’re very specific questions about the probate law of Florida, so unless there’s a Doper on the boards who practises probate law in Florida, you’re not likely to get an answer.
The second reason is that the flavour of your questions suggest that you’re asking because of a real situation, not some hypothetical. Lawyers won’t answer those sorts of questions on a message board, because our codes of conduct say that we are only to give legal advice about a specific issue after getting full account of the facts from the client, and only if we are familiar with the law in issue.
Your best bet is to consult a lawyer who practises probate law in Florida. If you don’t know of a lawyer who does, contact the state or local bar association - they normally operate referral services, and can give a list of lawyers who practise in a particular field in your area.