I dated a guy in spring of 2015 for about 4 months. We got on fairly well and I generally found him attractive. I dumped him when I realised I couldn't be arsed to spend £20 and 1.5hours to go see him. He was stingy and although he was away a lot with work, he didn't make the effort to see me. I give these reasons just to show it was a fairly mundane not-particularly-compatible 4 months.
Anyhow, over the past 5 years, I've heard from him randomly out of the blue every few months. I suspect it's whenever he gets dumped or is off in some hotel being lonely. I have replied once in the past 4 years when he messaged me (again) on Linkedin. (I only replied as he made some comment about us scamming a restaurant when in fact I'd just made them honour a deal they told us had been in place). He Whatsapped me in the early days then must have deleted my number. He's messaged me on the dating app saying he'd come across my profile again (I hadn't used it for 3 years but they emailed me). He sent me a lengthy email earlier this year apologising for hurting me. I think he's referring to the fact we hooked up once again in 2015 and we were all cuddly in bed and then he told me about some other ex he'd been sleeping with. I can't say I was hurt. In his email he said I'd been the best girlfriend he's ever had and he apologised for being emotionally unavailable. He said he was foolish to let me go (and saw great significance to some drunk guy who once took a shine to me and told him he should never let me go). I ignored the email. He's now started following me on instagram and recently messaged me to say he'd commented on a picture of mine. Turns out he'd also messaged me on Valentine's day saying 'Merry Christmas'. 
Anyhow, the point is do I keep ignoring him or do I put him out of his misery and tell him to stop being so desperate? Surely it can't be healthy to be pining over a fairly non-descript 'relationship' for 5 years?