I don't generally suggest or endorse products in this blog but when I run across something that really stands out in terms of 'bang for the buck', I like to share it.
Need to lock users out to perform upgrades or maintenance? Check out Rockton Software's Dynamics GP Toolbox. http://www.rocktonsoftware.com/products.aspx?p=12
For a very modest price, the Toolbox provides the ability to lock users out and much more.