Making individual addresses fail:
If you wish to block all e-mail going to an e-mail address, simply create a forwarder for that address that forwards to:
Please note that you must have the colons on both sides of the word fail for it to work.
Making the default/catchall address fail:
This method also works if you wish to make the default address (also known as the catch-all address) fail. Putting :blackhole: as the default address will make all e-mail addressed to an e-mail address/alias (that you have no made a pop3 account or forwarder for) fail.