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.