A squirrel is different, no body owns the squirrel. Cattle are owned by someone. You can’t shoot it or get it ‘removed’ yourself. If your in Australia (since I’m not sure since we don’t have squirrels here). You call the DPI (department of primary industry). They organise a ranger to collect from Local Lands council who impounds the animal (not shoots).I think that shooting the cattle is not the best option. Just speak with your neighbor again, and explain her about this problem, try to find a compromise. In case this doesn't help, you have the right to call the police, and ask them to call an eviction company that will deal with the bull. I mean this is what I personally did when I had problems with squirrels, I called a squirrel removal company, which helped, and I solved the problem peacefully. You decide in the end, but trust me if you will shoot that bull you will have problems with the law.
Anyhow I’m not going to repeat the answer. It’s already written on this thread.