You did not say which version of Acrobat you are using, but in Acrobat 8 do this to make the signature field read only when it is signed:
Signature field's properties > Signed tab > Mark as read only > Just these fields > [name of your signature field]
If the person who signed the document has Acrobat Pro, you could use the flattenPages() method, which lets you specify the page(s) to be flattened.
this.flattenPages(0) //flattens page 1
this.flattenPages(0, 2) //flattens pages 1 - 3
But if they have only Reader, they can't use flattenPages().