The LED's in the Intel® SSD 750 Series Add-in card cannot be disabled.
They are used to monitor the status of the drive. The activity of the 4 LED's is described in page 33 of the Intel® SSD 750 Series Product Specification.
I was afraid that was the case, thanks for the confirmation. It would be very nice to be able to disable them for "lights-out" operation...
Thank you for you comment, we have taken note of your feedback so it can be considered for the future.