。
Don't use violent language.
Literal
Violent [attributive-な] words [object-を] use-[must-not-てはいけない].
~てはいけない ('must not do X') is the standard negative obligation — stronger than ~ないでください. 乱暴 is a na-adjective ('violent / rough / rude'), taking な when directly modifying a noun. 乱暴な言葉 literally 'rough words.'