This issue occurs when there are any traces of the marketing effort in the REVENUE and BATCHREVENUE tables in SQL.

To resolve the issue:

1.  Run the following two scripts (replacing the value in red with the correct GUID for the marketing effort):

select * from revenue where mailingid='2bd7078e-4c8d-4092-b975-1b6647444b96'

select* frombatchrevenue where mailingid='2bd7078e-4c8d-4092-b975-1b6647444b96'

2.  Then, delete all references to the effort found in these results.
3.  Refresh and delete the marketing effort.