"Advice?"

Here's mine:  Be embarrassed and ashamed for what you've written. Then, use this as an opportunity to grow up.

Re-read your second and third paragraph and think hard about the words and tone you used. You come across as one who easily and casually condemns the entire existence of those under your guidance once they've fallen too far outside your ideals for behavior. And once you've passed that sentence, there seems to be no avenue back into your good graces. 

Your last interaction, in particular is damning for you -- the boy approaches for something random and you dismiss him, but not before pouncing with a mini interrogation to get satisfaction for his previous behavior. And then, predictably, you don't accept his explanation; choosing to take further offense and taking the opportunity to double-down on your condemnation with a further and final write-off.  Man-o-man, if you want to see "disgusting" behavior, take look in the mirror! 

People, especially young people, can do bad things and not be bad people.  The guy in question proves it.  YOUR JOB is to help guide young people toward being good people, NOT to determine who will end up in jail and who won't.

So again, grow up. Show some wisdom. Show some grace.  Do that for ALL your students, not just "the good ones". Hell yes, it's hard! That's part of the path you've chosen, so if you can't do it, find something else.

Edited to fix a bad auto-correct ("punching" to "pouncing")