Can't Truth in Advertising laws be use to nail liars like this?
The serious answer is no, because of something called the Entanglement Doctrine. Extremely briefly summarized, what this means is that the establishment (of religion) and free exercise (of religion) clauses of the First Amendment prohibit courts from interpreting church doctrines.
In other words, if you were going to sue these people or prosecute them, it would have to be in court, obviously. To find that there was fraud, a judge or a jury would essentially have to make a finding of fact (that means that they decide, based on the evidence, that something has been proven) that the Book of Mormon is not true. A court can't decide religious questions.