1. Impanel a grand jury, and indict Kerry under the Logan Act. It's broadly written, and fits like a glove.
2. Smile as he gathers top-flight legal counsel.
3. Prosecute him.
4. Expect to lose: the justice system in Washington DC will find a way to exonerate him.
5. Appeal to the circuit courts. Let them strike down the Logan Act.
6. Immediately demand the dismissal of the charges under Robert Muller of Trump associates talking to foreigners. Don't contest them.
7. Smile as the case against the administration is gutted by the Swamp for the benefit of a Swamp Creature like Kerry.
Justice. Cuts no ice.