Repairing cars

Either unload the resource that offers repair or add a block in the resource itself, to allow for certain users. IsPlayerAceAllowed

Basic Aces and Principles Guide

FiveM native Reference

Example:
server side…

RegisterServerEvent("permsRepair")
function callRepair(param)
  if IsPlayerAceAllowed(source, "administrator") then
    print("param: "..param)
    -- Trigger the client event to initiate repair ...
  else
    print("That user has no perms...") -- use TriggerClientEvent("chat:addMessage", source, ... here to send to player
  end
end
AddEventHandler("permsRepair", callRepair)

in your server.cfg you would add the ace perm for the person you want access to that command
add_ace Admin administrator allow