What’s shown in the page masthead is actually the page “excerpt” using the_excerpt().
This is intended for using the actual post_excerpt value as a summary of the page content.
There are (at least) three approaches here. You could:
1) Add a short custom excerpt for the page in admin that summarizes the content and will appear in the header. This is the “suggested” approach for cosmetic and usability purposes.
2) Use CSS to hide that area of the page…
.page-masthead .excerpt {
display: none;
}
NOTE: You can scope this using body tag classes, etc.
3) Add a filter to the excerpt to return false…
add_filter('the_excerpt', '__return_false');
NOTE: This may have collateral damage to other parts of the page (e.g. meta descriptions) so please scope accordingly.