Starting from Liferay 6.1 the Marketplace portlet has been provided in the control panel to download and install plugins inside the portal.
But once logged in to the Marketplace, with the specific portlet, it's impossible to make logout; let's see how to remove Marketplace credentials.
The solution is quite simple but it requires some database changes; just execute the following SQL query:
delete from ExpandoValue where tableid in (select tableid from ExpandoTable where name = 'MP');
Once executed the SQL query clear the database cache and, maybe, restart Liferay.