1. In Allocation Management, open the Allocation Set
  2. Select the Source/Destination tab
  3. Right-click the row that corresponds to the line in the Allocation Pool and select Delete Row. Click Yes to delete the row.

    Note: Do not click the red X in the icon list, it deletes the entire allocation.
  4. Save and close the Allocation Set
  5. Open the Allocation Pool and delete the line

Note: If the allocation is an Indirect Cost Allocation, copy the pool and remove the account/project from the copy.  Then create a new allocation and set it to use the new pool.