I understand the fencing thing now and it sounds like he could use some upgrades on his fencing.

I fully understand why you don't want his bull with your heifer but I don't think you can dictate how a neighbor utilizes their land, unless they are not complying with regulations or there is a binding contract in place so I'm inclined to think that you don't want him to use that particular pasture rather than the bull is not supposed to be there. Having said that, you are well within your rights to have him dealt with according to the law and I think it would be wise to gather accurate information now. I don't know of any place that doesn't take into consideration that things happen once in a while but provisions are certainly in place for habitual offenders.

If it happens again, take pictures, take pictures and document appropriately but stick to facts rather than emotion. If you have to take the matter to small claims court for restitution, you need to have your ducks in a row.