You can do this in v2008 and v2010--not sure if it was implemented in v2006. Could be worth checking the version histories.
You present your same html pages as custom items rather than html items. That way you won't automatically get the continue button. That is simply a matter of selecting "Custom Item" for the items you already have marked as "HTML" items.
The trick with using regular HTML under the guide of a custom item is using a password. Add a password parameter to the item such as "cat"--this would look like (xcat) in the parameters field. The trick here is that if you know of or can create a posted variable named "cat" in the HTML, then the custom item will end at that point. One easy way to do this is to find out what variable names are posted on the last HTML page your subjects will see or submit. Enter one of those values as a password and you're good to go.
For example, try using www.empirisoft.com as a custom item by entering that in the filename field. Then enter (xindex) in the parameters field. You'll find that MediaLab will cut out of the custom item if you click on any of the navigation links at the top because they all post a variable named "index".
Hope that helps.