How do I make sure that my little brother classmate in highschool that I met at work is not talking to my younger brother? Who do I report?

I don't want him to get involved. I don't know how he has his number.

He is still under 21 in college.

How do I make sure that my little brother classmate in highschool that I met at work is not talking to my younger brother? Who do I report?
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