Quinine (from tonic water) helps some people:
http://www.nhs.uk/Conditions/legcrampsunknowncause/Pages/Treatment.aspx
However, having had plenty of problems with cramp after exercise in the past myself, my own solution would be to
increase the frequency of exercise. I found that if was only refereeing one football match per week (with minimal training in between) I'd nearly always end up with cramp. If I refereed three or four matches per week, and ran a couple of half-marathon distances for training as well, I never got cramp. (Fortunately you won't have to start running half-marathons though. Just see the daily exercises in my link).