Use content-filtering software to block P2P websites.
Configure routers and firewalls to close the ports used by file-sharing software. P2P services may appear, disappear and change IP addresses, so this requires vigilance.
Monitor network traffic for certain types of files such as .mpegs. This could also raise a flag if an outsider were using your network to traffic illegal files.
Set up access privileges so that employees can’t download software. Install software or change permissions as needed.
Limit the amount of disk space employees have. Dole out extra space only when you know it’s for a legitimate purpose.
Consider installing auditing software, such as Lighthouse from Cohesiant, that tracks what kinds of software are installed on employees’ computers.