You can add following code in your theme’s (child theme’s if any) functions.php
add_filter( 'fep_current_user_can', function( $can, $cap, $id ){
if( 'delete_message' == $cap )
return false;
return $can;
}, 10, 3);
This will prevent user to delete their messages from front-end. So they will only be able to send message what is set in that limit page.