Do not hold this against most Protestants. The allegation came from an extreme Protestant cult called "The Way." There are many wonderful Protestants out there that are seeking truth just as we are and, yes, there are some that are borrowing from the information put out by the anti-Catholics. These extreme cults which are an embarrassment to many Protestants and are drawing from the most outlandish misinformation. You will see statements like the one from David Childress coming from cults like Jehovah's Witness, 7th Day Adventists, 7th Day Baptists (not to be confused with Baptists), The Way, and other independents. They are all drinking from the same anti-Catholic trough.
Yes, there are main line denominational Protestants who get caught up in this anti-Catholic sentiment; however, keep in mind they have become victims as well because they are spreading bad information without realizing it. And so I would only tell the Catholics simply to invite them away from these sources.
Anathema Catholic answers: Anathema
"In the early Church, anathema was used interchangeably with excommunication and to refer to unrepentant heretics. It then came to mean the severest form of excommunication in official church writings. When the authority of Rome was split in the Great Schism between Eastern and Western churches in 1054, an anathema was issued by Rome against the Eastern Patriarch who then issued another one against the cardinal who delivered it" https://www.catholic.com/magazine/print-edition/anathema - Jimmy Akin
The word anathema is in both Catholic and Protestant Bibles. For example, in the King James Bible, it says "If any man love not the Lord Jesus Christ, let him be ANATHEMA" (1 Cor. 16:22). If I were to insist that the King James Bible is pronouncing curses on other people, I believe that at least a few non-Catholics would be angry with me and rightly so. It is also wrong for anti-Catholics to say that the Church is pronouncing curses because of the use of the word anathema.
In terms of the person David Childress, he evidently is unaware of the different definitions of the term anathema. I believe that he was simply not aware that his own tradition was misleading him. Was he aware of the fact that the word anathema is also in the King James Bible? I don't think so. Did he believe that the King James Bible was pronouncing curses by the use of the word "anathema?" I don't think so. Was David on the basis of ignorance actually culpable? He most certainly was not fully culpable because he believed what he was writing, not realizing that his understandings were in error.
As for our response to David, even if we consider David as our enemy, we need to respond as Jesus asked us too. Jesus said to love our enemies. He also said to pray for our enemies and bless those who CURSE you!