I have a couple of suggestions that might help. I would recommend a two-pronged approach.
1) Reach out to those who make the changes and let them know nicely that while you appreciate their enthusiasm to help you with the profiles you manage, you are trying to only document source-verified evidence or whatever evidence you feel is appropriate, and if they can prove their changes, then they are more than welcome to participate. Or something along these lines, to let them know that you would value their input IF they have valid inputs.
2) If these mythologies exist in the internet (and many do), then I would document them on your profiles so that it makes it clear that you are aware of their existence and that they are clearly not proven and shouldn't be added without additional source evidence. Something along the lines (in the biography) of "John Smith has been noted as his father on many unsourced family trees, however, because there is no source documentation to prove this, he is not being added as father at this time. Further research is required." This may help, provided they read through the biography, so that maybe they won't try to add "John Smith" anymore without asking about why that note got added.