Delhi airport offers visitor's a visiting ticket allowing them to have access to their lounge.

Rail platform tickets are offered in many countries.

You may google and see, how this problem has been fixed.

See, when there is a visitor's ticket, a few will only visit the airport. This in fact acts as a barrier to the uninvited visitors.

However i have to agree, that despite the ticket facility, many will be there waiting outside in the sun and not willing to buy a lounge ticket.