People would mostly like to hear from an old friend, unless something bad had happened from the past. Or at least they would be polite and act like they really like to hear from you (even though just to disappear again).
For those who don't even try to be polite, and you're sure your friendship didn't end with a fight, then ignore and never be bothered with them again. They don't even have a common thing that is called "good manner".
For those who be polite and then disappear, just let them go. Life changes. They aren't like how they used to be. You aren't like how you used to be. They might like you in the past, doesn't mean they'll like you now.
And one thing is that if you don't want to look crazy in their eyes, then don't act crazy. I don't mind someone from the past randomly contact me, but I do mind if they act like those years of gap aren't there, like we're still best friends. I used to change clothes with my best friend, talk to her about boys when we were completely naked. That doesn't mean I will do that now.
I had a best friend in elementary school, we then went to the same junior high-school but in different classes and life kept us apart. We were still friends somehow, hanging sometimes, talking when we pumped into each other at school. It was when we reach senior high school that we were totally separated. neither of us "officially ended" friendship, it just naturally happened.
It was around 15 years ago, and recently we accidentally came across each other. I was happy at first, to meet my old best friend. But then she started to act like we are still best friends ( which we aren't now): hugging, leaning against me...(we're Asian and these are things we only do with people close to us, like families or, well, best friends), texting me about random things...
I stop contacting her, and she dropped by my aunt's store, asking for my numbers since she change the phone and lost it. My aunt didn't know so she promised I would contact her back. I did, just because of that promise, but then she was just as overly attached as before.
So I cut off with her. I did felt a little bit guilty though, I thought maybe she was lonely and just wanted a friend, and just didn't know how to do it. And then she tried the "losing phone numbers " trick with my aunt again and swept my guilt away.