They won't stop because they are either online grifters making money from this tit, or use this stuff as an extension of their personal, political or social identity. A lot of the folks going hysterical online are people with a frankly psychotic axe to grind against the royal family as whole.
You also cannot logic and rationalize people out of what was originally an irrational, unhinged, emotion-based thought process. Mostly, a dupe confronted with evidence they were stupid or wrong to a catastrophic degree digs deeper into their irrationality, or constructs an even more fantastical set of beliefs to explain the discrepancy. Princess Diana post-death attracts emotionally unhinged nutbags to the rate she almost THE litmus test for being unhinged. Chemtrails come a distant second.
People who are prone to deep conspiratorial thinking are just lost souls looking for meaning or control, some are mentally ill to a degree that need treatment, and grifters are just grifters who will just pivot to another set of nonsense. The point of it is eventually to encase the entire world in the grand conspiracy and sell people the idea that they are the clever ones for seeing it and being set apart for the 'sheeple' or equivalent dismissive term.
Having once - years ago - been part of a simple online prank to convince someone their idiot conviction that Sandy Hook was a hoax, and seeing the prank - which was meant to demonstrate that 'evidence' such as online timestamps can be easily altered by people with the correct permission such as a forum owner, become 'evidence' for a group of Sandy Hook Hoax conspiracy loon despite telling them that is was indeed a prank, taught me a lot. People buy into this stuff out of emotion and disordered personalities, not out of logic. So you cannot back them out of it via logic either. Most of them will just cycle through this stuff until they're dead or get treated for their mental issues.