Even if the spider doesn't respect robots.txt, the act of indexing is really obvious -- your webserver does not have to respond to their HTTP Requests if you do

Even if the spider doesn't respect robots.txt, the act of indexing is really obvious -- your webserver does not have to respond to their HTTP Requests if you don't want it to. It's not as facile as 'lock your doors' -- instead just not answering when they knock.