They make riser without LED's:

The 009S risers only have a small 12V and 3.3V LED, not the obnoxious disco LEDS's the newer risers have.

https://www.alibaba.com/product-detail/009s-Plus-Riser-6-Pin-Inferfaces_1600368709100.html?



The 006 risers have no LED's at all.

https://www.alibaba.com/product-detail/6-Pin-Powered-PCI-E-PCI_1600356511417.html?