Problem: As a user I start contributing in an app, and I don't know when I have to stop.
Solution: Give a tip after 5 submitted tasks that he can stop whenever he wants.
I would leave this to presenter apps -- what this suggests to me is that we need to pass into presenter apps information about the current user and the their activity in this app.
Also this relates to giving people badges -- this would be in PyBossa rather than the presenter. Something like a popup after you've done 10 items saying: Great Job, for all your hard work you've received the "Contributor, Bronze Level badge for this app". Then show these on user pages. However, what this, already detailed suggestion shows is: We need some proper new user stories!!!
This could be considered solved as issue #145 and #175 have been added to the PyBossa interface. We may need to stress in the UI that app creators should encourage their volunteers that you can stop whenever you want.