VPN Running on Centos 6 - Need to ban Torrenting

Essentially, using only the packet filters, you can't, at least not without blocking everything and then allowing through the bare minimum necessary. You need DPI filtering to block torrents properly.

There are many changes you can make to cause bittorrent downloads to be more troublesome to complete. These include blocking access to popular tracker URLs, closing standard BitTorrent ports, and even Deep Packet Inspection to identify BitTorrent packets.

However, these sort of changes will not prevent BitTorrent from working, just make it work harder. In short, most methods of blocking BitTorrent traffic do not effect encrypted BitTorrent connections.

In most cases, the best option would be to implement a tracking system for the users of your service, so that you can identify who is using BitTorrent and take action on that user's account.