WordPress: Register taxonomy hack WordPress. In the register_taxonomy function, add : $args = array( 'meta_box_cb' => 'tp_taxonomy_radio_meta_box' );

tp_toolbox.php

<?php
/**
 * Display taxonomy selection as radios
 *
 * @param WP_Post $post
 * @param array $box
 */
if( !function_exists('wearewp_taxonomy_radio_meta_box') ) {
	
	function wearewp_taxonomy_radio_meta_box($post, $box) {
		$defaults = array('taxonomy' => 'category');
		if (!isset($box['args']) || !is_array($box['args']))
			$args = array();
		else
			$args = $box['args'];
		extract(wp_parse_args($args, $defaults), EXTR_SKIP);
		$tax = get_taxonomy($taxonomy);
		$selected = wp_get_object_terms($post->ID, $taxonomy, array('fields' => 'ids'));
		$hierarchical = $tax->hierarchical;
		?>
		<div id="taxonomy-<?php echo $taxonomy; ?>" class="checkdiv">
			<?php if (current_user_can($tax->cap->edit_terms)): ?>
				<input type="hidden" name="tax_input[<?php echo $taxonomy ?>][]" value="0" />
				<ul class="categorychecklist">
					<?php foreach (get_terms($taxonomy, array('hide_empty' => false)) as $term):
						$value = $hierarchical ? $term->term_id : $term->slug;
						?>
						<li>
							<label class="selectit">
								<input type="radio" name="<?php echo "tax_input[$taxonomy][]"; ?>"
									   value="<?php echo esc_attr($value); ?>" <?php checked(in_array($term->term_id, $selected)); ?> />
								<?php echo esc_html($term->name); ?>
							</label>
						</li>
					<?php endforeach; ?>
				</ul>
			<?php endif; ?>
		</div>
		<?php
	}
	
}

/**
 * Display taxonomy selection as checkboxes
 *
 * @param WP_Post $post
 * @param array $box
 */
if( !function_exists('wearewp_taxonomy_checkbox_meta_box') ) {

	function wearewp_taxonomy_checkbox_meta_box($post, $box) {
		$defaults = array('taxonomy' => 'category');
		if (!isset($box['args']) || !is_array($box['args']))
			$args = array();
		else
			$args = $box['args'];
		extract(wp_parse_args($args, $defaults), EXTR_SKIP);
		$tax = get_taxonomy($taxonomy);
		$selected = wp_get_object_terms($post->ID, $taxonomy, array('fields' => 'ids'));
		$hierarchical = $tax->hierarchical;
		?>
		<div id="taxonomy-<?php echo $taxonomy; ?>" class="checkdiv">
			<?php if (current_user_can($tax->cap->edit_terms)): ?>
				<input type="hidden" name="tax_input[<?php echo $taxonomy ?>][]" value="0" />
				<ul class="categorychecklist">
					<?php foreach (get_terms($taxonomy, array('hide_empty' => false)) as $term):
						$value = $hierarchical ? $term->term_id : $term->slug;
						?>
						<li>
							<label class="selectit">
								<input type="checkbox" name="<?php echo "tax_input[$taxonomy][]"; ?>"
									   value="<?php echo esc_attr($value); ?>" <?php checked(in_array($term->term_id, $selected)); ?> />
								<?php echo esc_html($term->name); ?>
							</label>
						</li>
					<?php endforeach; ?>
				</ul>
			<?php endif; ?>
		</div>
		<?php
	}

}

/**
 * Display taxonomy selection as select box
 *
 * @param WP_Post $post
 * @param array $box
 */
if( !function_exists('wearewp_taxonomy_select_meta_box') ) {

	function wearewp_taxonomy_select_meta_box($post, $box) {
		$defaults = array('taxonomy' => 'category');
		if (!isset($box['args']) || !is_array($box['args']))
			$args = array();
		else
			$args = $box['args'];
		extract(wp_parse_args($args, $defaults), EXTR_SKIP);
		$tax = get_taxonomy($taxonomy);
		$selected = wp_get_object_terms($post->ID, $taxonomy, array('fields' => 'ids'));
		$hierarchical = $tax->hierarchical;
		?>
		<div id="taxonomy-<?php echo $taxonomy; ?>" class="selectdiv">
			<?php if (current_user_can($tax->cap->edit_terms)): ?>
				<?php
				if ($hierarchical) {
					wp_dropdown_categories(array(
						'taxonomy' => $taxonomy,
						'class' => 'widefat',
						'hide_empty' => 0,
						'name' => "tax_input[$taxonomy][]",
						'selected' => count($selected) >= 1 ? $selected[0] : '',
						'orderby' => 'name',
						'hierarchical' => 1,
						'show_option_all' => " "
					));
				} else {
					?>
					<select name="<?php echo "tax_input[$taxonomy][]"; ?>" class="widefat">
						<option value="0"></option>
						<?php foreach (get_terms($taxonomy, array('hide_empty' => false)) as $term): ?>
							<option value="<?php echo esc_attr($term->slug); ?>" <?php echo selected($term->term_id, count($selected) >= 1 ? $selected[0] : ''); ?>><?php echo esc_html($term->name); ?></option>
						<?php endforeach; ?>
					</select>
					<?php
				}
				?>
			<?php endif; ?>
		</div>
		<?php
	}

}