If a GS ventures out of the circle then your defence should block her when she tries to re-enter.  In order to do that your defence needs to be on the circle edge and  use small side steps to stop her re-entry.  Your defences will need practise at this, as it is easy to stick knees and shoulders out which of course will be contact.  Once they have the footwork sorted remind them that they need to keep their eye on the ball so they can still read the play, while minding their own player.