data/creaturescripts/scripts/
login.lua
Remove :
LUA:
player:sendOutfitWindow()
If you want remove "Set Outfit" or whatever is called in Pop-Out menu while you right click on character, you need to search deeper in server sources and if you want to remove display of that option in this menu you need to use OTClient and simply remove it from menu.
You can even remove packet about it to not hack this with any packet sender or something.