The International Ban on Duelling is a ban that, seen as it is international, probably was passed by the International Confederation of Wizards after 1994. During the Winter of 1994, the British Ministry of Magic seems to accept the terms and attempt to convince other wizarding goverments to do the same, as they were trying to convince other countries to agree to this Ban. Percy Weasley, working for the Ministry, was trying to convince the Transylvanians to sign it. 
This ban most likely refers to the more lethal "duel to kill" variety of duel, as opposed to the sporting variety.
- Harry Potter and the Goblet of Fire (First mentioned)