I agree with Natahlia. If you have a lunge whip keep it out so you have a sort of space bubble. If they start to drift in you can take the whip and just poke their shoulder/belly with it to remind them to stay out. Good luck!
Also say out when he or she does that
My horse does the same thing, if you have a lung whip you hold it towards him or her so it warns him or her off. Hold the whip so it is pointing towards its tummy :) hope it works