Oh come on. You really have to ask? A person like that would not be a "neighbor" to me. Call the sheriff/police and record a trespass against him/her. The next time it happens, he/she goes to jail. You better have proof though. Like a survey of your property and game camera photos. See a lawyer for damages. As before, you need proof. Sounds to me like he wants a confrontation. Give him one but don't leave traps or you'll go to jail.
Just spraying herbicides on another's property would cause you all sorts of trouble here. Unfortunately in the OP's case, the landowner is ultimately responsible for maintaining the contract he signed to not mow the area in question