As discussed in the following thread:
Check out the FREE Document Lock Plugin .... it was just released on 12/21/2010.
JIVE NOTE: Working with key supporters of this request, we've delivered a new capability called Reserved Documents as part of the Spring '14 Jive Release in Cloud and coming in Jive Custom 8. Reserved Documents are our people-centric approach to complex document workflows: simple, social, and out of your way unless you need it. You can read more about this core capability in Sneak Peek: Deep Dive for Reserved Documents.