There is something off about this person. You really should give absolutely no attention back. Ignore, block, ignore, block... It is not rude to not accept a facebook friend request from someone so distantly connected to you.
I wouldn't encourage this person with any attention whatsoever. You want them to just move on. There is no benefit to responding, and any discouraging message would probably be broadcast far and wide amongst the old acquaintances of that era (I'm thinking that due to the strange picture thing).