Users are prevented from doing this when Amortization Journal Entries have already been created for the Amortization Schedule.
1. Delete the Journal Entries found in the Amortization Schedule
2. Edit the source transaction (Vendor Bill/Journal Entry)
3. Remove the Amortization Template, Amortization Start Date and Amortization End Date from the transaction line item.
4. Save the source transaction
User will no longer see the Amortization Schedule under Lists > Accounting > Amortization Schedules and in the source transaction's line item level.
*The resolution above is also applicable to Revenue Recognition Schedules.