This is a tricky situation and I'm very aware a few people on this forum can sympathise with very similar problems.
As you admirably say they must have been closely affected by what happened. However, it's unreasonable for them to use you and your business as a punching bag especially 40 years down the line for issues that they need to deal with themselves too.
Sadly, your problem is not unique, there are stories here how 'random people' have brought up someone's past and it's affected their job, where they live etc. Unhelpfully, seemingly not a lot can be done about it. In my view, an authority figure should make contact with the person and make them realise the repercussions of their actions - I say authority figure because I'm not sure whose responsibility this should be, but one would assume it to be police as more of domestic-type enforcement.
Hopefully, it will blow over and this person will find something better to do with their time than to trash your business online, if they continue, it could be grounds for harassment. Maybe something has happened to them that's triggered the original incident and they're taking it out on you. It's none of your concern, of course, but I'm trying to make sense of the 'why now'.
Otherwise, I don't know if you're able to operate under a different name? That might be difficult if you operate face-to-face, I guess. I'm using a different name to 'hide' my past, my original name is unusual so very easy to Google. It's worked so far, but I'm very aware it's a fragile state, as soon as one person links my old name with new name... well, I try not to think about that too much. Sorry this is probably not much help, but if it's really problematic give the people at Unlock a call and talk through it, they're always very helpful. Good luck.
Fighting or Accepting - its difficult to know which is right and when.