Open Windows command prompt as Administrator, and type:
powerfg lastwake (press Enter)
Windows will then list the event that caused the computer to wake.
Then type the following command:
powerfg -devicedisablewake (exact name as that listed above)
For example, if the last wake was caused by a USB controller, and the above name is as follows, type:
powerfg -devicedisablewake Standard USB Host Controller (Enter)
This action will shut down the event causing the computer to wake from sleep mode. If this advice doesn't fix the problem, then post back. Maybe someone has a better solution.
Edit: There may be other ideas here:
In case you need additional ideas.