Megan, can you tell us more about the use case? What are you trying to accomplish? What kind of task?
Potential Use Case:
Trigger campaign that assigns a SFDC task to a lead when they click an email. Task subject is "Clicked Email: Followup"
Is there a way to setup the smartlist to exclude any lead that has already logged the activity of having a SFDC task created with the same name "Clicked Email: Followup" in the previous week?