|I believe this will require a 3'd party application.|
If this is a WRT54GL then you could try a 3'd party firmware to see if it's capable. I have Tomato running on mine but it doesn't have any throttling capabilities. Perhaps one of the other's does, you'll have to check yourself. For a look at Tomato and several other firmwares for the 54GL, have a look at this link to the Tomato site. Scroll all the way down to the bottom of the page to see the other versions availabe.