The simple answer is “just don’t let story points stack.” You can only carry one, and you get at least one per session (beginning or end, doesn’t matter), with more for when they do something cool. Then, there is literally no reason to hang on to them. You know you have another coming, just spend it.

I also let my players hand out story points. “Whoa, Bartrok, that was really cool. You should have a story point for that.” Yeah, the first session, they went a little overboard, but I was pleasantly surprised by how well that worked.