In v5.0.3, you can edit the template file:
/ZimbraWebClient/WebRoot/templates/share/App.template
look for the ID, "skin_container_logoff" and just add "style='visibility:hidden'" to the table cell.
Assuming you've installed ZCS via source, making a change like this means you'll have to redeploy the web client. Under /ZimbraWebClient, run "ant prod deploy"
Goodluck
