It happened again towards the very end of the Gran Colombia game I was playing. Because of the horrible/challenging start I had being on a tiny isolated land mass without much to work with and only one CS, I wasn't quite sure in the early game what kind of victory was my best option. I had diplo and religious victories disabled so I opted to go for a science win. I didn't really gain momentum and a lead until the Renaissance era. I had maintained friendships and alliances with Georgia and Babylon throughout, but I got bored while waiting for my space missions to do their thing so I went around and liberated some free cities to pass the time.That caused civs who were unfriendly towards me to become friendly. However, they were either at war with or had denounced my allies so I wasn't interested in making friends. I only did it because I had units sitting around and free cities in areas that were easy for me to get to and felt like passing the time. The Ottomans wanted to declare friendship but at that point I was a handful of turns away from winning. So I said "No, stop asking". Sure enough, 5 turns later he asked again, to which I replied "No, stop asking", and then I got the science win before he had the chance to ask again.
I'm not sure if save files will help but this only happened at the very very end of the game. I don't think it matters who you play or what settings you pick, it seems like it's a feature that was meant to be a good thing but is broken and makes the AI more annoying than ever when they come to you every 5 turns like they have a goldfish memory and you didn't just tell them to stop asking.