I remedied this by drilling a hole in a base using a 3/32″ drill bit and inserting the same size rare earth magnet in the hole. Can only drill in the recessed area (close to a point of the base) of the underside.
Simply drill your hole, add a drop of super glue, insert the magnet, and the base is done.
Next, I used a piece of paperclip (a big one) and after drilling a hole in the bottom of the ball, glued the bit of paper clip into the base of the ball on the underside out of sight.
Now, the ball sits on the base without falling off and being annoying. Easy fix!
EDIT: As another poster on the Mantic forum pointed out, magnets in the bases could cause attraction between models when they are all bunched together. An easy solution would be to magnetize the ball and put the paperclip in the base. Something I didn’t think about!