two options i can think of.
1. using a shared variable passed fromo the sub report to the main report and keying off that. It will be a little tricky due to needing to have it under the sub report section
2. I think the easier process is to, in the sub report return the a string where ISNULL(activityfield) then 'inactive' else ''
and conditionally display that in the subreport header and conditinolly suppress all of the rest of the subreport in reverse