Although i don't really get what you want this for, since this will stop all players from entering the portal, but here you go lol
Lua Code:
function onStepIn(cid, item, position, fromPosition)
config = {
pushback_position = {x=, y=, z=}
}
doTeleportThing(cid, config.pushback_position)
doSendMagicEffect(pushback_position, 10)
end