So you can do this one of a few ways, but I would generally recommend doing them all as they aren't without fault since this isn't the easiest thing to identify.
1) Application restrictions:
The firewall is capable of identifying a number of p2p applications already through app-id. Look through applipedia and see what applications you actually want to block and deny the applications in your security rulebase as you see fit. For example, you can already block 'bittorrent', 'tor', 'tor2web', and a number of other applications through app-id.
2) URL Filtering:
If you are licensed for URL-Filtering, there is an entire 'peer-to-peer' category that is dedicated to sites that allow p2p transfers. I'd recommend blocking this category in its entirety as it prevents the user from going to things like utorrent.com or piratebay.
This doesn't make it impossible to download things via a p2p network, but it does give you the ability to say that you are denying the traffic based off of best-effort.
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