Take the strapholder nut off the threads, you will see two small holes in the threaded part of the jack. Use a small screwdriver or rod through these holes to stop the jack spinning as you tighten the nut.
If it won't tighten enough you will have to adjust the nut from inside. Attach string through the 2 holes in the jack, undo the nut and washer completely and find the wires connecting the jack through the soundhole. Gently pull and you will deliver the jack through the soundhole, (make sure your string doesn'r disappear) Adjust the internal nut and use te string to pull the jack back into place. Reassemble washer, tighten the nut and put the straploc back on. Job done.