How can I show a message to visitors of a blocked user account page in Panels?
You could try to use the Rules module for this, using an approach that is similar to my answer to the question "How to create a redirect to point to the user&#39;s id?", in which you use values for the_old_path and the_new_path that fit your needs here, i.e.:


Change all occurences of the_old_path to the path of the user profile.
Change all occurences of the_new_path to the path of your custom variant.
Add a Rules condition to check if the user account is blocked.
Fit the message being displayed to fit your needs. If you don't want such message to appear, then just delete the line containing drupal_message in the rule.