QoS with wireless on WRT54g

Hi, first post here.

I have a Linksys WRT54g running Linksys 4.20.7 firmware.
There are 2 computers and a VoIP box linked via ethernet, and a third computer via wireless. It's the third comp that's causing the problem...

I need to either limit the bandwidth available to the 3rd comp, or use QoS to make it the lowest priority on the network. (As you may have guessed, it is used exclusively by a 13yo with Limewire)

I have used the QoS in the router with some success; it works well by giving the VoIP box high priority...

BUT I can't limit the wireless system, and it's driving me crazy...

Can someone please help out with the best way to limit this wireless connection? I can't connect to it via ethernet.

TIA, Will