If I understand your situation correctly, what if you created another score field (in Marketo only) that would dictate whether or not they could get a task? Something like "Task Score"
For example, in your flow for every campaign where a task is created:
1. Create Task
2. Change Score "Task Score" +1
3. Wait 24 hours
4. Change Score "Task Score" -1
Then, on every Create Task flow step that you want, add a choice prior to the task being created that says IF Task Score is greater than 0, do nothing. This way, only those who haven't had a task created in the past 24 hours will get another task.
If you want to only limit your daily 8PM run to only create a task if one wasn't created that day, only apply the Create Task choice to that campaign.