The only thing you can really do is not issue admin accounts to employees, but that can hinder productivity. It might also be helpful to use an enterprise anti-virus software; as much as it pains me to recommend AV for a Mac, my coworkers sure can load them up with crap. An AV solution can prevent that.
FWIW, we do issue admin accounts to employees on their assigned Macs and rely on both AV installed locally and network based AV to keep us protected. Everything else is either local server based or cloud based; ie., easy to enforce least privilege regardless of client device.