Hi,
As your storage type is having stock so, the system would be having storage units belonging to that storage type. so, i don't think system would allow you to deactivate the SU management at st. type level.
Even though if you find a way to change in the system how would you do that physically?
I think you would need to create a dummy (virtual) storage type - activate SU for that and move all the stock in your sloc to this dummy sloc. Then deactivate the SU check and move the stock back.
Regards,
Sushant