Is there any way to archive completed tasks?
Currently, there’s no specific feature Archive in Kanbanchi. However, there’re two options to organise Archive (as much as Backlog).
Option 1. Create a separate list, name it Archive and move done cards there (either by drag-and-drop or in Card Details)
Option 2. Create a separate board for these purposes. You can add this board to a set of boards in one tab to quickly switch between them and move cards from one to another (you can also save it for quick access). To move done cards to Archive board follow instructions on moving cards between boards.