There is an option in pixelmon settings to disable the appearance of wild pokémon per generation; but not in the way you ask. (You can't create exceptions) The only shortcut I can think of (without being much of a server expert and based on my experience) is to completely remove the wild pokémon and...

I was unaware of the existence of the file, thank you very much for the advice and sorry for the inconvenience.

NPC Editor ->Add an interface in the npc editor to modify the products/prices of each pixelmon shopkeeper. ->Improve or create an advanced interface on the pokemon trade NPCs, assigning the following parameters: - Assign a nature, nickname, skills, abilities, moves, ivs/evs, gender, size, equipped ...