Hi @Steph_30,
I came across this unanswered question while doing a community cleanup.
First, I'm not clear if you're re-using the same course shell year after year or not, but if you are, I'd encourage you to make a new one for every year (or semester). On the outcomes side, in the account outcomes area, I created a group called "archives" have been moving the "old" outcomes to that area. It helps people from selecting the old ones for new assignments, but you still do need to make sure only new ones are being used in new courses (a somewhat manual process). I do believe Canvas will lock you out from deleting the old outcomes as that would break courses and reports where those outcomes were used, so you probably do have to keep them around somewhere...
I hope this helps a bit, even though it's a very late answer.
-Chris