The amazing Doctor Zymes will crush their puny souls!
You can even use it when your plant is in flower, with no worries.
If you don't have, or can't afford Dr. Zymes, grab a few cigarettes and rip them up into some isopropyl alcohol. It will pull the nicotine out of the tobacco. Then, dilute it a little with water and let loose. It should give them nicotine poisoning in under a minute, killing them.
Good luck!