Sorry. AFAIK, Starter Edition does not support loan accounts. If Quicken won't let you add a Balance Adjustment transaction to set the balance to 0.00 … have you tried marking the account as closed (Tools Menu / Manage Hidden Accounts)? That's supposed to do all the cleanup (deactivate account, set balance to 0.00, etc.) without deleting any of its transactions and causing other headaches, e.g., with transfer transactions.
It would be interesting to hear wha the result would be if you try to close it. Since the Starter Edition doesn't support loan accounts, when you downgrade like that it is supposed to make them read-only. How “read-only” they are is anyone's guess since I haven't heard of anyone reporting on this.