Oh dear, yet another mare being bred, by people who know absolutely nothing about horses, let alone breeding.
The OP has been given very good advice. I'd like to add here, that keeping a stallion is a HUGE responsibility. When you remove him, make absolutely sure, you have him in an are with stallion-proof fencing. If you don't, then he will probably escape by jumping or pushing a fence down and injuring himself in the process. Very large vet bill or dead horse.
If he does escape and mares belonging to others are in the area, he will likely get over the fences and breed their mares also. Could results in a law suit or your having to pay all fees connected with the keeping and foaling out those mares.
Breeding is not for the inexperienced. It takes much knowledge of the background of the horses used, what genetic tests must be in order for their breed, good and safe foaling area and a solid bank account, in case the mare has trouble during the foaling process.
If the mare was only just bred, then call your vet to give her a shot to abort. Geld the stallion immediately. Do not put him with the herd for at least six weeks after he is gelded.
Until then, take care in handling him. There was probably a good reason why he was free.