Untested

local component = require("component")
local computer = require("computer")
local alarm = component.os_alarm
local entity = component.os_entdetector

allowed_player = "SuPeRMiNoR2"

while true do
  clear = true
  players = entity.scanPlayers()
  for name in ipairs(players) do
    if name ~= allowed_player do
      clear = false
    end
  end

  if clear then
    alarm.deactivate()
  end

  if clear == false then
    alarm.activate()
  end

end