Should a Church call out False Prophets who claim to speak with God's word?
Most churches would call out such false prophets out in one way or another.
At the end of His Sermon on the Mount the Lord warns the faithful against false prophets, comparing them to wolves in sheeps’ clothing. The "dogs" and "swine" the Lord just spoke about have a depraved way of life that is obvious; they can only be repellant, and are thus not as dangerous to believers as false prophets. False prophets present their lies as truth, and their rules of life as godly ones; one must be sensitive and wise in order to see the spiritual danger they represent.
"Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves. Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits ye shall know them. Not every one that saith unto me, Lord, Lord shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity." - (Mat. 7:15-23).
In the Catholic Church, usually the local bishop will make some kind of official announcement on such a subject, which would I turn be published in the local Catholic information sites, both in paper and electronic media. Sometimes Rome will make such a call, if the magnitude merits the situation or notoriety.
Here are examples of visionaries judged to be false in the Catholic Church. Although it deals more with visionaries, the steps are the same for someone claiming to be a prophet. Many of these visionaries predict certain things to happen.
Some individuals have been pronounced against by name, e.g., Vassula Ryden, and the Little Pebble, William Kamm. Vassula has been condemned twice by the Holy Office (the Congregation for the Doctrine of the Faith), on the grounds that her revelations do not come from God, and because they contain errors against the Faith.
You hear people say: "But her writings are so spiritual and so beautiful!"
I agree; possibly 99% of Vassula's messages are in conformity with the Catholic Faith — but that is just how the devil operates to deceive pious Catholics.
It is the 1% that does harm.
A poison apple is mostly good apple — but will harm you nevertheless. The devil knows he cannot mislead devout Catholics with outright heresy, but he can appeal to their piety and then subtly plant errors within.
In any case, there has been no approved revelation in the history of the Church where God took someone's hand and gave messages by writing with his pen. But you do find handwriting messages given at séances and séances are condemned by the Church as a practice of the occult against the law of God.
I have seen one pious magazine defending Vassula by saying that Cardinal Ratzinger never signed the statement against her printed in L'Osservatore Romano. A man I know sent them the official statement from Acta Apostolicae Sedis, the official Vatican gazette, which has the cardinal's signature at the bottom, along with that of the bishop secretary of the Congregation for the Doctrine of the Faith.
Unfortunately, the editor of the magazine had neither the humility nor the honesty to print a correction in the next issue.
Another example: the alleged apparitions in Garabandal in northern Spain, in which four young girls alleged that the Virgin Mary appeared to them from 1961-1965. The response of successive bishops of the diocese of Santander has been uniformly negative, and the present Bishop Vilaplaua has concurred with this verdict.
Despite this, there are a number of active associations supporting Garabandal.
A simple case of disobedience to lawful authority.
This is only one of a countless number. There's Montichiari in Italy (1947), Necedah in the United States (1949), Palmar de Troya in Spain (1968), Bayside in the U.S. (1970), Dozule in France (1972), and hundreds of others — to say nothing of all the alleged visionaries and locutionists past and present, such as the Irish lady, Christina Gallagher, and many other poor deluded souls.
Mrs. Gallagher's messages, in part, read like a frantic worried woman lamenting the state of the world.
There are plenty of frantic worried people, lamenting the state of the world, who are good Catholics — but the Blessed Virgin from Heaven does not talk like them, in such a human, earthly, fretful fashion. To attribute such talk to Our Lady is an insult.
"Have visions; will travel" — such publicity seekers are not to be believed. Genuine visionaries fly from publicity. They do not go around with photographers and camera crews. They submit to investigation by Church authorities; but they do not have publicity agents. - Apparitions True and False
As for statements attributed to the Pope (such as "I heard that the Pope told Mr. Smith after Mass in his private chapel that he believes in Garabandal and Bayside" or "The Pope told Jane Doe that he could go ahead and print that condemned book") — no one is entitled to act on such gossip. The Church is governed by publicly promulgated statements — not by hearsay and personal communications.
Rome publishes it responses for all to see.
The Popes may choose to show their approval of certain revelations, after the decision of a local bishop or conference of bishops, by speaking of them, or by placing a new feast in the liturgical calendar, or by visiting the places connected with them.