If I am understanding you correctly you are referring to the email address that appears when you begin to type an email address in the email window.
These email addresses are stored in the COMMON_SETTINGS table in the Quality Center project. To see what email addresses will auto-populate when the user begins to type you can look in the database to see what email addresses exist for the CSET_VALUE where CSET_CATEGORY column is equal to MAILED_USERS. This query should display the email addresses: Select * From COMMON_SETTINGS where CSET_CATEGORY='MAILED_USERS'
This Query can be executed from site admin by expanding the project in the left hand column under the Site Projects tab and then putting the SQL into the text box at the top and clicking Execute SQL.
If you wanted to actually get rid of the auto-complete form data for the project you could clear the entry in the database.
**##Note## This will remove all auto-complete email addresses in the QC project, also it is a good idea to temporarily deactivate the project and backup the database before making a change no matter how small**
The SQL to clear the autocomplete data in the project: Update COMMON_SETTINGS set CSET_VALUE='' where CSET_CATEGORY='MAILED_USERS'
This will clear out all of the auto-complete form data for the emails in the project, new emails addresses that the users type in will start repopulating this entry in the database.