From the PSR FAQ at svpworld.com:
How to turn off effects.
1) Press [FUNCTION]
2) Press [E] (Reg.Sequence/Freeze/Voice Set)
3) Press [NEXT] twice (Voice Set)
4) Uncheck ''Effect'' for or any/all voice sections (according to your needs)
5) Press the [DSP] button to turn Off DSP
Now when you change voices, the DSP will not turn on.
Regards,
Michael