According to the books, President Snow used poison on anyone who posed themselves as a threat. He was known for using this method at dinner parties. The death would be blamed on bad oysters or something of the sort.
Nightlock was known in District 12 and other districts to be deadly poisonous and could kill you with just one berry. It was used by Katniss Everdeen and Peeta Mellark to win the 74th Hunger Games and kill off one of the final tributes, who Katniss nicknamed Foxface. After the games, they were also used by President Snow to poison Seneca Crane after letting the two tributes live. This was accomplished by locking him in a room with nothing but a bowl of nightlock, then was forced to eat it.
No further use of nightlock has been confirmed neither in book or film, apart from being the passcode for the Holo. When the word "nightlock" was said three times, the Holo would self-destruct.