You can disable autostart at any time using PROUTIL EPOLICY MANAGE, as shown:

proenv>proutil t1demo -C epolicy manage autostart disable -Passphrase
OpenEdge Release 12
Enter the key store passphrase for database t1demo:

Manage autostart disable has been selected. (15523)
This command modifies encryption access control in the Keystore file. After 
successful completion of the command, the Keystore file must be backed-up. 
(15518)
proenv>

Observe that -Passphrase is added to the command line to override the autostart user account configuration.