The RIAA does not exist in the 9500 as a discrete board that can be simply removed.
In the 3500 deck I believe the job can be done by cutting one leg only on C4 and one leg on C12. This should disable the op-amp. Now either connect the pins together as described by Peter or move the yellow wire from pin 7 to pin 6 in the plug.
You already know the simplest option, remove the pins and plug it into Tape 2!