How can i remove the Meth Buyer NPC or the DropOffPoints?

Both of them get saved under GarrysMod\garrysmod\data\zmlab\

1. MAPNAME_MethNPCs.txt
2. MAPNAME_MethDropOff.txt

You can remove them which removes them from the Map.

Easier way to remove them individually would be to remove the Entity with the Remover ToolGun and resave them using !savezmlab in the Chat. This will override both files for the Map.