How do I get it through to him?

I was going to choose the ignore him option BUT if he's crazy (like me) being ignored just fuels the fire to keep trying and chasing you. I think you need to tell him seriously and honestly why you shouldn't get back together and say no and THEN ignore him. If he comes around just tell him no over and over. He should eventually get the point and move on. I think just flat out ignoring him could cause problems.

I voted "other" and here is why. You said you don't "THINK" you should be together, but the real question is "Do you WANT (or not want) to be together?" And why? Were you happy? Did he treat you bad? Are you with someone else? This is something you need to know, not think, and I am not sure you know. You need to do some soul searching so that you know in your own heart what you want, and if it IS no, then tell him to F OFF!

no clue

I would just ignore him.

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You've tried to be polite, so the next time tell him that it has no chance of working and to not talk to you again, then ignore him

Tell him to eff off THEN ignore him

Frankly, I'm having a similar but not as direct issue recently and ignoring is the path I went for. My ex and I broke up on bad terms... Basically he just stopped talking to me after an almost two year relationship. There were a lot of unsolved feelings and things we needed to talk about, but I had to move on and now I'm in an incredible relationship and don't really feel the need to "talk it out." However, he frequently messages me lately to ask if I want to talk because HE wants to talk HIS feelings out now--too bad, so sad cowboy. You had your chance and now I've moved on!

Ignore him.