Hi Richard,
To say "Don't get involved. It's none of your business.", expressions below may be used.
However each sentence may be translated in slightly different way.
1) 네가 상관할 일이 아니야.
= It is not something you can get involved.
2) 네가 알 바 아니야.
= It is not something you have to know.
3) 너하곤 상관없는 일이야.
= It is not something related to you.
4) 상관하지 마.
= Do not care.
5) 참견하지 마.
= Do not care. / Do not talk anything about it.
FYI, 4) and 5) expressions may sound more rude than 1), 2) and 3).
Hope it helps.