I found a thread here that has a link to this site with pics and scans of the TSB from Toyota on disabling the DRL's. maybe that will help.

I know for certain this is easily doable on the Vibe, which isn't far off from your Prizm. Just be careful around the "DRL relay" as it's more of a module that controls all sorts of other things besides DRL's too... ;)

I simply pulled the DRL relay.