weed out - Fig. to remove someone or something unwanted or undesirable from a group or collection. (Fig. on removing weeds from the soil.) We had to weed the less productive workers out one by one. The auditions were held to weed out the actors with the least ability. I'm going through my books to weed out those that I don't need anymore.

bad apple - The full form of this proverb is 'one bad apple spoils the barrel', meaning that a bad person, policy, etc, can ruin everything around it.

If you prefer not to mix two idioms then just weed out the corrupt.