You need to either delete the maintenance internal file created by WPT (you can view it's path in the "plugins" panel in WP admin, under "integrated", "embedded" or something similar), or stop using WPT to manage that website.
I might be wrong, but I think all sites managed through WPT have that...
Issue - Feature request - Annoyance - Large number of customers and broad audience
UPDATE 1: added additional reference.
I am an employee of a company that uses Plesk as their hosting management panel; and I have a private account on the same server.
However, being an IT professional (and...