Have you ever looked at the handle on the Petzl I'D?  

On the notched side (where your thumb from your left hand would sit when descending) you will notice a screw that accepts an allen key.  I found a 2.5 size allen key works to unscrew it.  

Then, if you look at the moving side plate (the big gold plate)  you will see a hole left of the safety gate (the black part you move to open the I'D) which you can put the screw in to lock it, if you so choose.

Now, why would you want to do something like that?  

Well, I have a few ideas, but from what I've read, according to the technical notice, this could be used in a rescue kit — something that you don't want people messing with and undoing.

Kind of a tamper-proofing it.

One thing I did notice... if a rope is installed and you have the device locked like this... 

Is that the handle does not go fully back to the "store" position.  It kind of sits out not making it such a narrow piece of gear...

If you crank on the handle, I don't know what would happen...?  You could possibly break the handle, or damage your rope (crushing it)?

Might want to mention that to whoever is using the device in this manner.

Now, with everything... There are pro's and con's.  You probably wouldn't want to do this if it was on your harness and you used it to convert from ascend to descend.  It jams stuff up and would be a pain to get stuck like this...

There are a few other holes that the screw looks like it could go in to, but those are for added friction, called an auxillary brake (instead of using a carabiner redirect).

I'm wondering when else you would want to use this to lock the device?

Maybe you don't want the person operating it to open it while descending?

If you have any reasons why, post them below.

*** BONUS TIP ***

When connecting a carabiner or opening the safety gate to remove the device from a carabiner... position it so the carabiner is nudged up towards the top of the device.  By moving it ever so slightly, it opens easily and freely.  Maybe less wear on your device and carabiner?