There is a new feature in Oracle General Ledger R12 (12.1.3), which lets you define an additional qualifier called Third Party Control on your natural account values. You can set the Third Party Control qualifier for your accounts to be only used by Payables (Supplier) or Receivables (Customer), or prevent any manual entries (Restrict manual Journals). As General Ledger Super User, navigate to Setup > Financials > Flexfields > Values, select the value set with your GL accounts, and specify how the accounts can be used.
As is often the case with new Oracle functionality, there is a catch. As of today (August 2012), the Third Party Control does not apply to the journals uploaded from WebADI, which is how most (if not all) companies create their “manual” journals. There is an existing Oracle Support note ID 740851.1 acknowledging this fact and referring to a pending enhancement request (Bug 8289661) requesting remedy of this issue.