|There are devices that can be used to block specific applications but they're expensive and I suspect cost more than you really want to spend to stop this.|
I would do what ijack says. Institute an "appropriate use policy" at work with regard to how the the work computers and network can be used. Make it clear in the policy use of any point-to-point sharing, or torrent type software is against policy and make an example of the first person you catch by firing them and then check each and every other computer at work the next day and pass around a company wide email stating you are now monitoring for such abuses.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.