OBSTA PRINCIPIIS or "Nip it in the Bud"

From: Black's Law Dictionary

Latin: Withstand beginnings; resist the first approaches or encroachments. “It is the duty of courts to be watchful for the constitutional rights of the citizen, and against any stealthy encroachments thereon. Their motto should be ‘Obsta prineipiis.’ ” Bradley, J.. Boyd v. U. S., 116 U. S. 035, 6 Sup. Ct. 535, 29 L. Ed. 746.

The poet Ovid once wrote:
"Principiis obsta, sero medicina paratur, cum mala per longas convaluere moras."

"Withstand evil when it starts, it is too late to apply a remedy when the evil has grown strong by long delay."