Not saying you should but you could:
* While they're working, go in front of their boss and say some shit like "You need to stay away from my 16 year old little sister" or I'll have police come and deal with you.
* Could accuse them of rape
* Plant a shit ton of illegal drugs in their home/car and secretly call the police to frame them.
* Straight up destroy their car/house
* Post their photo online and say that they're a child predator, leak their phone number, address, etc.
* Go to a shit ton of scam websites and enter their personal information so they get a shit ton of scam calls and emails.
* This is fucked up but you could knock them out cold with a baseball bat and cut their dick off