Tenant harassing tenant

[Kyle D.]

I have a situation at my duplex where the tenant next door is harassing the other tenant. This all started a year ago. But has escalated severely as much as the one tenant is now harassing the neighboring tenant by doing all sorts of things to her. So basically this tenant was originally in section 8 when they moved in but 2 years ago they have opted off section 8 program. So now they have been living there for 2 years without section 8 and pay rent every month no problem. I have no lease with them so there on a month to month verbal lease. So I am getting tired of being bothered about this. So I was gonna draft a 7 day breach of verbal lease letter and mail it to them. Is this the right course of action? What should I put in the letter? Is there any letters already made that I could use? Any body have experience with this situation? Thank you