Just manually enter an IP and subnet mask (probably 255.255.255.0) from your network's range into your PS3 (maybe 192.168.1.50 or 192.168.0.50), and put a bogus gateway address (like 18.104.22.168). No need to mess with MAC address, the wire (it's built into the NIC anyway).
You have to manually set a FIXED ip address in your ps3 network settings, you need the real gateway address which is your routers ip address usually 192.168.0.1, you only need to enter bogus dns addresses and disable auto sign in to psn.
If you are un-sure about your gateway address and stuff, on your comp go to the start menu and enter cmd then open and type ipconfig/all, it will tell you that information there.
To stream films in a home network its best to give your pc a fixed ip address as well, its a pain in the arse to do for the first time but its better long term.
I'd check the router admin pages, there'll probably be an option in there where you can turn off the WAN connection for a few devices (or IP's) failing that as natepig says giving the PS3 a bogus DNS address should take care of it.
You can either assign a false default gateway on your PS3, or block internet access in the router admin pages. Either way will work for you. Here's a random link for further explanation (from Google): forums.anandtech.com/archive/index.php/t-1921326.html
You don't need to give your PC a static IP since Windows and Linux (not sure which OS you're using) recognize the PS3's DLNA, and will route requests just fine LAN-side using MAC addresses (layer 2 OSI).
OP, just choose a method that works for you and let us know.
Last edited by moja; 04-21-2011 at 09:01 PMReason: Automerged Doublepost
For streaming with tversity and ps3mediaserver, fixed ip addresses on both sides is always best. The gateway must be the routers ip address for streaming to work and the bogus dns addresses is all that is required. Please try not to confuse people looking for help with irrelevant computer jargon.