Tenant Has Not/Will Not Sign Lease

[Matthew Slaughter]

Hey BP Community!  So here's the short end; Purchased a triplex a couple months ago.  Utilities have been pretty finicky so I finally have everyone moved to a flat fee, which not being on a flat fee caused some hairs to raise with the middle tenant.  That problem solved.  

However, she still has not signed her lease and I think she is doing it out of spite to provide a feeling of control, IMO.  There are two people living in the unit, one has signed, the other hasn't.  We have language in the lease that states you cannot have a guest for longer than 7 days without signing a lease.  In my eyes, they have breached the agreement.  Can I use the lease language to let the flatmate, who has signed, know that they are in breach?  What are my landlord options?