If anyone ‘believes’ they are speaking to anyone departed they are believing a lie because they “want” to ‘believe it. Of course what they choose to believe isn't going to change the ‘reality’.. that if you engage in practices God says not to you're ‘bound’ to run into all sorts of visuals in your mind..and out which the enemy of men's souls captivates people with.
People have varied reasons, in their own minds, why they bypass God and reach for “something else”.....King Saul did just that when he wasn't getting the answers he wanted to also.
I don't think you'll convince many catholics do set aside their “belief” in speaking to and through departed people. It’s way too embedded in their psychic from years of indoctrination....and this on the other pillars they've built to hold it up and vice-versa. If any one of those blocks falls the whole thing will come crashing down around them....and why they run around salvaging the bits God keeps chipping away at.
Its going to happen to them either now when they have a chance to repent or later when it's too late.