Keeping druggies and loiterers out of building

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I took over managing a building in Downtown Boston and I have a major issue with drug users loitering and using in my building. They leave drug paraphernalia and today a needle! I can only think to install a buzz in system which might be very expensive, unless anyone has done it in expensively for a 7 floor commercial/residential building? Or hire a security guard. I took out hallway doors and considering adding more lighting to make it less likely for someone to hide in the stair wells. We have cameras but they ignore them. Calling the cops every day seems like a huge waste of time. Anyone have any helpful ideas? Greatly appreciated.