Hint -- if you don't have any, look for a small tube of Loktite blue 242, available at hardware stores. Just add a dab (not a whole lot, you don't have to entirely cover it by any means) to the cap
threads, screw it back in, wipe off any excess.
The cap will still be removable, but it will take a conscious effort, gripping the knurled edge.
(generally handy stuff, I used it to tighten up bolts on my snow rake) (Loosen the top screw slightly before loosening the cap, if you ever need to unscrew it -- that keeps the wire from being stressed. Just tighten the screw after the cap is retightened.)