Prayers to God - for anything - are always welcome to God. God can handle all of them. As long as a person is praying to God, there is no harm. God will handle all that needs to happen, whether through actions, teachings, or whatever. To "warn" people not to pray for pretection against, or the abolition of, any evil is - at the very least - pride. At worst, it is actively seeking to protect that evil from the results of the prayer.
Maybe you should follow your own advice, and worry about your own cross, rather than presuming the authority to get between people and God. As they say, fools rush in where angels fear to tread. Daring to mess with other people's prayers is, IMHO, above your pay grade.
You might want to move this one to the Religion Forum.
Actually, creating a whole theology of principalities and an idea of fake spiritual warfare about how to deal with them is Apostasy.
And it’s not above any Christian’s paygrade to deal with false teaching.
We are called to be Bereans, and to test all things.
Thanks for the admonitions though.