Hello,
as you are using the legacy DB API (indicated by the usage of method prepareStatement), the "autocommit" mode is by default enabled (which confirms the assumption of Lars). You have to disable it by calling method setAutoCommit(false) before you do any changes, then you are able to use the commit and rollback methods (check JSDoc: Class: Connection $.db).
The "autocommit" mode is disabled by default in the new DB API (JSDoc: Class: Connection $.hdb).
Regards,
Florian