Auto-Check Category

Can’t see any hooks available for forcing or selecting a category automatically.

You could do it with jQuery however, it’s just not guaranteed to work for all users (they may have JS disabled as one example).

It’s still an option all the same.

add_action( 'admin_print_scripts-post.php', 'load_jquery_if_necessary' );
add_action( 'admin_print_scripts-post-new.php', 'load_jquery_if_necessary' );
add_action( 'admin_head-post.php', 'disable_and_check_cat' );
add_action( 'admin_head-post-new.php', 'disable_and_check_cat' );
function load_jquery_if_necessary() {
	wp_enqueue_script( 'jquery' );
}
function disable_and_check_cat() {
	?>
	<script type="text/javascript">
	jQuery(document).ready(function($){
		var required_cat = $('input#in-category-3');
		if( required_cat )
			required_cat.attr('disabled','disabled');
		if( !required_cat.attr('checked')  )
			required_cat.attr('checked','checked');
	});
	</script>
	<?php
}

Change the 3 for the category ID of the category you want selected and disabled.

For reference, this is the line that requires adjustment.

var required_cat = $('input#in-category-3');

You can set the default category in the Settings > Writing section. This will select the one you want by default, however it will allow them to uncheck it.

Copy all the code (into your themes funcitons file, or a plugin, after the opeing PHP tag <?php on a new line).

Update the the line i mentioned before to reflect the correct category ID (currently 3).