I think only transformPDF activity can do this. So you might require Output service for this feature.
Output Service is required to truly flatten. You can do some tricks with other services to make that fields on a PDF non interactive, but the PDF still think its an interactive form and appears like it.
If the form is a Static XDP based form (when you create the template you mark it as static or dynamic) then you can flatten it using Assembler. If the form is dyanamic, however; Assembler will try to call Output.