21 year old gelding trys to kill newborn foals??

We have a breeding farm with 8 broodmares which are bred to our stallion. The stud has a seperate paddock so that he wont fuss with the 3 geldings that run with our mares. The stallion has sired 2 foals which have not been bothered by our geldings but we recently allowed a lady to pay rent for her gelding to run with the mares as well. The third foal was born and I went out to check the mare and found the new gelding charging after the newborn and biting at her and trying to kick her. I ran out and got the filly before he could hurt her (and penned them away from the others) but I am worried about the next mares foal due in a few days now. How do we handle this???