Excessive bandwidth use could indicate a user who is downloading or uploading movies, torrents of pirated software or streaming video, or it could be malware on the machine. Either way it can impact network performance for everyone, could result in higher ISP monthly charges, and could put the company at risk from malware being brought into the network or at legal risk for copyright violations.
Check the user manual for the router that handles the network. There should be options for logging usage, and you can analyze the log to see which IP address is consuming the most bandwidth. Then you need to check the IP address against the client machines. Better quality routers will have extensive options for reporting and restricting bandwidth use, and restricting the Internet destinations that client machines can access.
If you don’t have access to router logs, you may be able to use a network sniffer on a machine in the network to capture information about your network traffic http://www.wireshark.org/about.html