I seem to have been asked this a number of times recently, so here is the answer.  To take a public folder offline, simply add it to your favourites and then enable synching of public folders.

Mark the public folder as a favourite (right click on the folder and select make favourite

Go into your account settings and ensure that public folders are enabled for caching

Tools / Account / Properties for the Exchange Account / More Settings / Advanced - then tick the box

Now, when you sync the favourite public folders will be copied too.  That is it

ttfn

David