As they attempt to enter the building spray them all with a 1:5 solution of cadaverine. Follow that with a couple of fire hoses to hose down the steps of the building to wash away the results of their retching, along with them.
I would recommend against that. My understanding of the law says that is "battery".
Far more would be achieved if they could be humiliated in a non racial way. Anything smacking of racism will do far more harm then good. It will be picked up by the national news services and turned into a club to beat opponents of illegal immigration.
Whatever is done, it would be better to be funny and completely avoid racism or even any such whiff.
As they attempt to enter the building spray them all with a 1:5 solution of cadaverine. Follow that with a couple of fire hoses to hose down the steps of the building to wash away the results of their retching, along with them.
OK, if not use flame throwers.