Heh, uh... I doubt it is legal to put a helicopter pad on a small building downtown...
The neighbors might complain!
So sure, let them do it then first time they land they get raided and loose ownership of the building, helicopter, and everything inside

Sure in the barrens maybe somebody could get away with it (the problems there are random people shooting at your helicopter just for fun, or stealing it), miles and miles from the nearest police officer, but not downtown even in a relatively crummy area.
But any building would have pretty high body. Vehicles don't win crashes with buildings until they turn into tanks/cargo ships, and even then they usually just tie.
Heh, if they don't hire an engineer to design the thing you could also just have the building collapse.