Nip the briar in the bud.
More French Proverbs:
One cannot be at the oven and the mill at the same time.French
Too late the bird cries out when it is caught.
French
Wolves do not eat each other.
French
A hundred years is not much, but never is a long while.
French
Adversity makes strange bedfellows.
French
He is a thief indeed who robs a thief.
French