First of all find all emails with allowed access with following SQL query in Plesk database:
MariaDB [psa]> select email from smb_users where isLocked=0;
Make sure that these are the addresses for which you want to deny access. If it is ok, run
MariaDB [psa]> update smb_users set isLocked=1 where isLocked=0;
I hope it will help.