Want to block P2P websites across domain

We have around 50-60 W2K/XP workstations (and 5 W2K servers) behind a SonicWall firewall. We do not have content filtering on the firewall and since we do not have any company policy on internet usage our IT director has no particular desire to upgrade our SonicWall package to include this.

However, one thing I would really like to do is to lock down file sharing sites like LimeWire, BearShare etc. I thought one idea might be to somehow use a script or Group Policy to deploy a Hosts file to each machine on the network with the following entries:

127.0.0.1    www.limewire.com
127.0.0.1    www.bearshare.com
etc...

Whether this would have been a good idea or not, the boss says no because the HOSTS file on a lot of machines are actually used by their quote vendors like Bloomberg, Reuters etc.

So basically my question is, are there any ideas for blocking certain websites on all machines when this cannot be done either at the firewall or by using a blanket HOSTS file?

Any suggestions most welcome.

Cheers.