has this been updated for 1.5.4
I have 1.5.4 and it works fine on my site.
Install the mod as suggested. Then go to your site and go to your IM (click on the "# messages" highlighted link at the top of the board where it says Hey, yourname, .... ). Then click on "preferences" and you should see the Ignorelist box where you can add usernames. Add a username then click save. Now when you view this user's posts you should see their post replaced with the message "<<you have blocked this message>>" or something similar.