One sure way to annoy your customers is to have a support blog and then prevent your customers from commenting there by presenting them with this message every time they try:

“Sorry, but your comment has been flagged by the spam filter running on this blog: this might be an error, in which case all apologies. Your comment will be presented to the blog admin who will be able to restore it immediately.
You may want to contact the blog admin via e-mail to notify him.”

Now, once or twice, you could forgive them for: mistakes happen. But this is every single time and is still happening after I’ve already complained about it directly to them.