Smoking outside office

I'm hoping somebody might be able to come up with some suggestions for me. Since a refurbishment at work a couple of years ago, we are having a bad problem with cigarette smoke coming into the building. There is an air vent behind my desk which we are unable to close off (expect by covering it with carrier bags). Smoke comes in though the vent and also into the main foyer through the entrance and exit doors. Although there are no smoking signs around the door area, smokers take no notice and ignore you when you ask them to move. The problem has been reported many times and very little is being done. Cigarette smoke is one of my asthma triggers and it also causes bad headaches (2 this week!). What can I do next please?!!

Thank you for any help