Neighbour's post holes on my property
Do you have a survey of your property that shows your building as well as the property line? Use it to determine exactly where the property line is. Does your neighbour have one? That's your starting point - establish consensus on where the boundary actually is. Hiring a surveyor to stake out the property line is your best bet.
Unfortunately we are seeing that this often isn't enough. Even with insurmountable official evidence in your favour you may find your neighbour continues their project. In this case your best bet is to contact a lawyer who specializes in boundary-related disputes.