Hi Paul,
What is the datasource of the report? If the report is based off of a Command Object, see if the prompts were created in the Command Window and you would need to delete it from there.
Otherwise, you would just right-click the parameter in the Field Explorer and select 'Find in formulas..' and delete every reference from every formula.
The prompt could also be just dragged and dropped on the report; to check this you would open the Report Explorer and scan one section at a time to see if prompt has been placed on a section.
-Abhilash