Blocking messages from a particular contact
I don't think you can with the default SMS app.

The 2 best alternatives, HandCent SMS &amp; GoSMS offer a feature called blacklisting - you can long-press hold a conversation and choose to add it to your blacklist. Such messages will be hidden (not deleted) and you won't receive notifications / popups.

You can also configure individual notification colour and ringtone for a particular user.