~$ sudo apt-get

Of all the things Plesk does well, this which would seem like common sense to include within the interface, cannot be done. To rectify this issue, you must update the default_domain field within psa.IP_Addresses.

mysql -uadmin -p`cat etc/psa/.psa.shadow` -Dpsa -e "UPDATE IP_Addresses SET default_domain_id = 0 WHERE ip_address = 'IPGOESHERE';"

Remove the WHERE statement if you wish to do a blanket default domain removal.

mysql -uadmin -p`cat etc/psa/.psa.shadow` -Dpsa -e "UPDATE IP_Addresses SET default_domain_id = 0;"

Plesk will populate psa.lockout in mySQL in the event of the lockout. Either of the following will clear the lockout.

mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -e 'TRUNCATE lockout'

mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -e 'DELETE FROM lockout'

Check /usr/local/psa/admin/logs/httpsd_access_log for brute force attempts.