There is no simple "pull out the fuse" trick to making it RWD. You need to either purchase an aftermarket or make your own locked centre diff. pop out the centre diff, attack it with a welder, stick it back in. In full rear you will find yourself snapping thin R160 drive shafts.
You can also install a DCCD centre from a 5sp RA box into your car and leave it open, which will perform pretty much the same thing.
If you'd searched the forums you would have found these threads
http://www.perth-wrx.com/vb/showthre...ar+wheel+drive
http://www.perth-wrx.com/vb/showthre...ar+wheel+drive
http://www.perth-wrx.com/vb/showthre...ar+wheel+drive
It also begs the question, why did you buy an AWD car just to make it RWD? Sliding 4 wheels is much more fun than 2
