You should probably give you pages a unique identifier and when the user clicks on 'Submit' or 'Next page' (whatever you have done) record in a database the user has completed that page.
user_name page_id is_completed
dmorley 1 1
dmorley 2 0
dmorley 3 0
As part of each page load, have the page check the database for the user and see if 'is_completed' is true. If so, have the page redirect to the next page or a holding page showing 'Sorry, you have already completed this section and must now proceed to the next page'.