How can I signal that I don't intend to follow up the items in my "Future Work" section?
I've never heard this about "Future Work".  Items that you put in this section are simply things that you are currently considering, and someone else who does them should at least cite your original work.  However, anyone who beats you to actually doing the work should be able to publish it.

If you want to make it absolutely clear that you do not intend to work on this material (and, you never know, you might end up going back to it later anyway!), simply call the section "Further discussion" or just "Discussion".