To disable the Windows + L (Windows 7, XP or Vista) you may perform the following registry hack found at howtogeek.com
Open up regedit.exe through the start menu search box, and then browse
down to the following key, creating it if it doesn’t exist:
On the right-hand side, create a new DWORD 32-bit value named
DisableLockWorkstation and give it one of these values:
1 – Disable Lock Workstation
0 – Enable Lock Workstation
The changes should be immediate, no need to restart anything.
There is also a downloadable .reg file that can also apply the update.
I have applied the hack and can confirm it does work on my Windows 7 machine