Your Kanbanery board performs at it's best with less content, and coincidentally, so does your team. We've created places off the board to store tasks that you don't need to think about either because they are done (the Archive) or because they won't be started for some time (the Icebox).
When your last column (probably "Done") is getting full, you can dump all of the tasks from it off the board and into the archive by clicking the archive all icon at the top of the column. Then the tasks are moved to the Archive, and you can review them using the Archive link in the main navigation.
If you have a task that you know you won't do for a long time, like a good suggestion or a very minor bug, you can get it off the board by clicking the Icebox icon on the task card. You can always visit the Icebox from the main navigation and move tasks back onto the board when you're ready to think about them.