The "lock down" depends on how obedient the users are :) Templates allow you to reassign key shortcuts, menus and toolbars (at least in 2003, I'm not sure about 2007), so in a way that takes away ability to mess up the document manually, but it is not that difficult to circumvent (so some user discipline is required). Instead, you could give them large buttons titled e.g. "Main heading", "Bullet point" etc. that would be linked to particular styles.

If a document is tied to a template, it should update automatically to the changes in the template whenever it is opened. Unfortunately, it is rather easy to modify the formatting (so it shows up as "Style + Arial", for example) - then of course manual overrides remain, even if the template is changed. Still, you could have a macro that reapplies the styles if needed (based on this tip):

http://www.elharo.com/blog/word/2005/12/28/word-tip-1-reapplying-styles/

Another option is to link documents in a master document - this allows to modify all of them at the same time without opening each inidividual document. The disadvantage is that each document has to be linked to the master manually, which might get tedious. Also, with a large number of documents this might become unwieldy.

You probably know that you can also explode tanks and trucks full of soldiers - all you need to do is to shoot the fuel cap (not that realistic, but still fun)...