My live-in girlfriend was paying rent and has stopped paying and I want to know what I can do legally to get her out?

In order to evict the tenant, the landlord must obtain a court order through a process called "summary ejectment".

If a landlord wants to evict a tenant who will not leave voluntarily, then they must file a Magistrate’s Summons and Complaint in Summary Ejectment in Small Claims or a/k/a/ Magistrate’s Court (you can contact them directly for the correct forms/procedures).  The tenant must be served with the Summons and Complaint by the Sheriff’s Office, either personally or by posting.  The Summons will state the date, time, and place for the court hearing. Small Claims Court hearings are informal, but both parties may have an attorney, present evidence, and subpoena witnesses.

The tenant may have defenses to the eviction action depending upon which basis for eviction the landlord sets out in the Complaint.  In your case I don't know what, if any defenses, your girlfriend could claim but she has the right to present any that she thinks that she has.  Then, either party has 10 days in which to appeal the magistrate’s decision to District Court for a new trial.  During this 10-day period, the landlord cannot make the tenant move.  The tenant may also be required to pay any undisputed past-due rent to the Clerk of Court.  

If the tenant does not appeal the magistrate’s judgment within 10 days or loses on appeal, then the landlord may evict the tenant by obtaining a Writ of Possession of Real Property issued by the Clerk of Court.  The writ directs the Sheriff to physically remove the tenant and his personal property from the premises.  The Sheriff, not the landlord, is the only one who can remove the tenant and/or his personal property from the rental premises.  The landlord cannot force the tenant to move.  In other words, do not use any "self-help" to get her to leave.  Things such as changing the locks or the like.  You could end up on the wrong side of a civil suit for wrongful eviction.