Mark,
You have several options, it seems you have touched on the basic ones nbar being one, and MPF. If this is a relatively small network in terms of users probably the easiest way is to directly contact the user to stop P2P file sharing downloads, but this approach is a quick non-effective workaround it, this would be if this is a small network 25 to 50 users where one could have more control.
1-Revice your MPF implementation to ensure that implementation is not meeting your needs, try exhausting this option, QoS rate limiting could be another option but what is the ultimate goal? Allow P2P file sharing in the company or completely stop non-work related downloads?
See Allow and Block the Traffic Through the Security Appliance
http://www.cisco.com/en/US/products/ps6120/prod_configuration_examples_list.html
2- Consider looking into other alternatives, if you have ASA 5510 or above you may want to look into CSC-SSM module :
CSC-SSM - Look at data sheets
http://www.cisco.com/en/US/products/ps6823/index.html
SSM module configurations example and file blocking
http://www.cisco.com/en/US/docs/security/csc/csc61/administration/guide/csc4.html#wp1002608
CSC-SSM Q& A
http://www.cisco.com/en/US/prod/collateral/vpndevc/ps6032/ps6094/ps6120/prod_qas0900aecd8040397e.html
3-NBAR would be another solution but you have indicated ISP managed router, perhaps placing a router between ISP router and your ASA5500 firewall will give you the management access to implement NBAR.
4- Websence is to be a very effective product
Rgds
-Jorge
Jorge Rodriguez