Love Speidel! Yeah, I'd close it up, and if you're worried it's going to carbonate (or more than you want at least) loosen the threads on the cap and bleed it off once in a while. But it sounds like you're fermented out. I ferment in a Speidel and rack to a corny to lager, and if activity has died off like that, I bet you won't ferment out more than 1 gravity point more (if any at all.)