The other day, I was creating a page template using the standard approach used in forcing a specific layout. However, when using page templates, this will work if they have not selected a specific layout on the page in the Genesis Layouts metabox. So you could either hide the Genesis metabox, or you can have the page template ignore any changes made in the layout metabox (less coding!).
The normal procedure is as follows:
However, this ignores any custom fields that may upset this. So since this is pre_get_option_*, the user can accidentally over-ride the "forced" specific layout. So if you don't want the user to over-ride it you need to go a step further. You can add one more line of code to correct and ensure that a page can never have a different layout. One way is to set the post meta to always be whatever you want.
// Force Page Layout for Page Template
update_post_meta( $post->ID, '_genesis_layout', 'full-width-content' );
However, the Genesis approach would be to set it via the pre site layout Filter.