At the moment, there's no way to archive tasks or projects. Our developers are working on a way to accomplish this, but it hasn't been released yet. I do not have an ETA on it's release but it is coming soon.
Any plans in the pipeline to create the Option of archiving Projects ??
I just created a network called "Archived Projects" and then exported the project that was complete to that network. Worked great. Next year I can just export it back to the original network and update the dates. Perfect.