Rob, we struggled with this in my subscription as well. We found that it became less of an obstacle as our implementation matured because people began to naturally ignore Accepted (or whatever end state you use) work items. We also noticed that our SAFe and scrum teams don't experience the problem in the same way that continuous flow teams do because they focus on stuff by timebox, which makes it easy to disregard the contents of past timeboxes.
Ultimately we've chosen to keep work items in place and not to archive anything except for defunct Projects, which we keep in a special section of our project hierarchy. We mostly do that instead of closing projects because the contents of a closed project aren't searchable, much as I wish otherwise. Closing a project would be a nice, natural archival method if it weren't for that limitation and there's been quite a lot of chatter about it in Agile Central Ideas. The most recent discussion that I chimed in on was at https://ideas.rallydev.com/ideas/D4060 . Hope that helps.