Well, I've done it several ways. One is to have a "Completed" thought on a per project basis (or at whatever you like), which after the "Action" items that I wish to keep are changed to a different thought type, then they can be moved under that "Completed" thought so you won't see them when looking at the actions.
Again, part of my strategy is to do something with the completed actions - keep is as reference, evolve it to a different task, or forget it.
If you want to keep all the tasks around for documentation purposes, then moving them to the "Completed" is a decent strategy.
This is definitely a painful area of the the brain - not being able to select or filter based on two or more dimensions is a limitation that makes task management difficult - for example, I can select one tag at a time and see all the thoughts tagged that way. I can select a Thought type and see all the thoughts in the plex with that thought type. I cannot, however, select a thought type and then filter by tags.
Basically, trying to manage tasks requires more work than it should.