Some suggestions…
Do you add Projects from Templates? If yes, perhaps the person was somehow added to the Template itself and that is why they are receiving the notifications. I had that happen a couple of times, where a person was receiving notifications on Projects that they would not have anything to do with, so I checked the Template itself, and for some reason, they were listed under the More >> People tab. So, anytime anyone used the Template to add a Project that person was getting notifications. (I had also double-checked that the person was not assigned to any of the tasks on the Template itself, so there wasn't a reason why they would be listed under the More >> People tab.)
Secondly, on a Template, check under Template Actions >> Project Sharing to see if the person is listed there. If they are and should not be, then you would want to remove them from there.
Hope this helps…
Terry Terry Hynd EBSCO Information Services