little disappointed that with 18 people having viewed my original question no-one has been able to answer it or to even write a reply. I can only guess that perhaps there is no way to lock the device which really is quite unsatisfactory
I looked at archos.com online version of your manual, and it confirms there is a lock using the on/off button, but it doesn't say how, so you can experiment.
Of the 18 viewers, perhaps none have your device. Your topic name didn't indicate what the issue was, and didn't even indicate it was a question. Perhaps they hoped they would learn something from looking at your post, or perhaps many of the hits were by search robots cataloging knowledge.
If you stuck to one topic and right now edited in a meaningful title like "how to lock?" you might better catch the eye of a person that knows. But the manual should have given you the clue of what to focus on, whether the feature works or not.