As mentioned several times (like in this treath)http://forums.b2evolution.net/viewtopic.php?t=986&highlight=delete+blog
it's not that easy to write code for that.
We know lots of people want it, but it will not be for rigth away.

Okay folks it's going to be in the next release.

That was a PITA. Let me share how insane this is:

	/** 
	 * Delete a blog and dependencies from database
	 *
	 * Includes WAY TOO MANY requests because we try to be compatible with mySQL 3.23, bleh!
	 *
	 * {@internal Blog::dbdelete(-) }}
	 *
	 * @param boolean true if you want to echo progress
	 */
	function dbdelete( $echo = false )
	{
		global $DB, $tablehitlog, $tablecategories, $tablecomments, $tableposts, 
						$tablepostcats, $tableblogusers, $cache_blogs;

		// Note: No need to localize the status messages...
		if( $echo ) echo '<p>mySQL 3.23 compatibility mode!';

		// Get list of cats that are going to be deleted (3.23)
		if( $echo ) echo '<br />Getting category list to delete... ';
		$cat_list = $DB->get_list( "SELECT cat_ID 
																FROM $tablecategories
																WHERE cat_blog_ID = $this->ID" );

		if( empty( $cat_list ) )
		{	// There are no cats to delete
			echo 'None!';
		}
		else
		{	// Delete the cats & dependencies
	
			// Get list of posts that are going to be deleted (3.23)
			if( $echo ) echo '<br />Getting post list to delete... ';
			$post_list = $DB->get_list( "SELECT postcat_post_ID 
																		FROM $tablepostcats
																		WHERE postcat_cat_ID IN ($cat_list)" );
			
			if( empty( $post_list ) )
			{	// There are no posts to delete
				echo 'None!';
			}
			else
			{	// Delete the posts & dependencies
			
				// Delete postcats
				if( $echo ) echo '<br />Deleting post-categories... ';
				$ret = $DB->query(	"DELETE FROM $tablepostcats
															WHERE postcat_cat_ID IN ($cat_list)" );
				if( $echo ) printf( '(%d rows)', $ret );
				
				
				// Delete comments
				if( $echo ) echo '<br />Deleting comments on blog\'s posts... ';
				$ret = $DB->query( "DELETE FROM $tablecomments 
														WHERE comment_post_ID IN ($post_list)" );
				if( $echo ) printf( '(%d rows)', $ret );
		
		
				// Delete posts
				if( $echo ) echo '<br />Deleting blog\'s posts... ';
				$ret = $DB->query(	"DELETE FROM $tableposts 
															WHERE ID  IN ($post_list)" );
				if( $echo ) printf( '(%d rows)', $ret );

			} // / are there posts?
			
			// Delete categories
			if( $echo ) echo '<br />Deleting blog\'s categories... ';
			$ret = $DB->query( "DELETE FROM $tablecategories
													WHERE cat_blog_ID = $this->ID" );
			if( $echo ) printf( '(%d rows)', $ret );

		} // / are there cats?
		
		// Delete blogusers		
		if( $echo ) echo '<br />Deleting user-blog permissions... ';
		$ret = $DB->query( "DELETE FROM $tableblogusers 
												WHERE bloguser_blog_ID = $this->ID" );
		if( $echo ) printf( '(%d rows)', $ret );
		
		// Delete hitlogs
		if( $echo ) echo '<br />Deleting blog hitlogs... ';
		$ret = $DB->query( "DELETE FROM $tablehitlog 
												WHERE hit_blog_ID = $this->ID" );
		if( $echo ) printf( '(%d rows)', $ret );
	
		// Unset cache entry:
		unset( $cache_blogs[$this->ID] );
		
		// Delete main (blog) object:
		parent::dbdelete();
				
		echo '<br/>Done.</p>';
	}