A filly named Pipsqueak

"Spark Nova, for the repeat offence of underage rape, you are sentenced to life in prison with no chance of release."

All light faded from Spark's face.

"Now, as for the crime of attempted castration, we have not had to deal with such crimes in over one thousand years, so we never saw a need to change this law."

Spark started to cry. "Don't do this! Please! NO!"

Luna's smirk fell into a blank stare. "Spark Nova, you hath wrought this punishment upon yourself." The princess used her magic and grabbed Spark's horn before breaking it, causing the unicorn excruciating pain. "I will not have any funny business while we do this, am I clear?"

Spark sobbed and nodded his head. His heart was pounding. He knew what was coming.

Luna levitated a knife and slowly brought it closer to Spark's testicles. She went as slowly as possible when slicing them, much to the unicorn's torment. Eventually, she cut the sack off, leaving the testes hanging.

"It is not often that we use this magic, as it is grotesque and incredibly dangerous, but this punishment truly warrants such measures." Luna said as she gripped the testes with her magic. Spark started to scream in agony as Luna slowly used her magic to implode his testes. Spark's cries did not affect Luna, however, as she continued to slowly crush the stallion's junk. After what seemed like ages, the balls finally burst in on themselves, and Luna finished her castration.

Spark dropped to the floor, curled up and continued sobbing and screaming in pain.

"I will leave you here until the guards come and escort you to the dungeons. I'm sure somepony who likes sex as much as you will make plenty of friends there." Luna said as she teleported herself to the other side of the door.

Guards came to pick up the sobbing mess of a stallion ten minutes later. Spark was sent to the Canterlot dungeons, put into stocks and was repeatedly molested by the inmates and force-fed/hydrated by the guards for the rest of his natural life. 58 years later, the stallion died of a heart attack.