Best learn and follow whatever your local estray laws are. In Texas, is illegal to haul estray to sale barn. We must call sheriff and he will send someone to get animal and deal with owner. If reported within 5 days think estray owner can be required to pay reasonable feed cost to you plus recovery fee to person who catches/hauls it. Would sure mention to appropriate person that this has happened repeatedly. Possibly request bull be trich tested so in case you end up with a bunch of opens there will at least be evidence if needed for recourse  In your fence situation, think adding a hot wire would be most cost effective way to handle this issue.