We have Shetlands and they are notoriously sexually precocious - on a visit to Shetland, almost every croft had one 15 month old heifer with a calf at foot.
Our bull has been running with our cows - two of which have heifer calves at foot that were born in May. Now I haven't SEEN him bothering them (I haven't seen him "bothering" the cows either so I hope he works in the dark
) but when the vet comes to PD the cows five weeks or so after the bull goes away, I'll get him to jag the heifer calves to bring them into season thus ending any unwanted pregnancies.
We had our original two heifers jagged when we got them at six / seven months old as they had been running with their mothers and the bull (their sire) and we've had the two heifers from previous years jagged likewise. It doesn't seem to have any adverse effect on future breeding but has to be done at the right time for maximum effectiveness - I think it's 98% effective if it's done at the right time. Your vet will be able to advise.