Currently, I have Postgresql disabled in Virtualmin. In the Module Config, under Enabled features for servers, "PostgreSQL setup and configuration enabled?" is set to No. Also under Webmin modules available to server administrators, "PostgreSQL Database Server (for database)" is set to No.
However, when setting up or editing a reseller account, the option for PostgreSQL database appears and is enabled by default. This seems different from the way the options appear when creating a new virtual server, such as the way disabled modules are not selectable on these pages.