Is there a way to disable a Linksys WRTP54G router so that users CAN NOT stream audio or video. I only want them to be able to send/receive emails and upload or download pictures within emails.
The default software for Linksys routers doesn't normally provide this level of URL filtering. You may find that iptables on DD-WRT (http://dd-wrt.com/) or even OpenWRT may work for you.
Netfilter (which we usually see as iptables) helps us filter on:
* Source/destination server or network * Service/Port * TOS/DSCP bits * Pre-defined traffic selectors for IM/P2P applications
A quick Google search offered up this:
http://l7-filter.clearfoundation.com/ L7-filter is a classifier for Linux's Netfilter that identifies packets based on application layer data. It can classify packets as Kazaa, HTTP, Jabber, Citrix, Bittorrent, FTP, Gnucleus, eDonkey2000, etc., regardless of port. It complements existing classifiers that match on IP address, port numbers and so on.
NOTE that the Layer7 filter HOWTO page's "actually doing stuff" section (http://l7-filter.sourceforge.net/HOWTO-kernel#Doing ) says "Instead of dropping packets you don't like, [use] Linux QoS" to limit their available bandwidth, which it then briefly describes.
(note that I have no connection to L7-filter or clearfoundation.com, nor 1st hand or 2nd hand knowledge of their product - just what I read on the 'net...)