Lots of correcting and time and put it on the leed when ferrets are on the floor. every time it goes to get the ferret pull the leed and say no , the more time you put in the easier it will get , try leting it see and smell the ferrets before you put them on the floor then sit on the floor with the dog for a good while and if it goes for the ferrets keep correcting it , as said once it realises that ferrets bolt rabbits and a ferrets not a rabbit the partnership should form ,