Did you install the Dell Display Manager software? If yes, open it, click the options tab, remove the check from PowerNap options.
Just to doublecheck...then on the right click Configure.
* Screen saver should be set to (None)
* Click Change power settings
* On the left, click "Choose when to turn the monitor off"
Turn off the display = Never
Put the computer to sleep = Never
* Click Apply- OK
* Press the monitor #3 Menu button to open the OSD (On Screen Display)
* Go to Energy Settings
Energy Smart = Off
USB = On
Power Button LED = On
* Exit the OSD (On Screen Display)