You would need to look at the language of the lease, carefully. There may be ways you can say she has broken the lease, such as having an overnight guest. I think possibly the easiest way might be if she does not pay her half of the rent, though there is certainly due process required.
Another way to do this might be to contact your landlord, and tell your landlord that you wish to modify the lease because of non payment of rents and strange guests. You would then need to notify her of some kind of intent to modify the lease at her last known address. Frankly, it would probably be good to both notify her at *your* own address and at her parent's address, and give her a reasonable 14 days to cure the problem (non payment of rent) or to object to the lease modification in writing.
The key in this is notice and documentation. That is because a future court will not want to think you just threw her out on the street. If she does not return, and you are able to get her off the lease by her default non-response, then you can legally rent to another tenant. Be careful not to sell her stuff; you may be able to store it and charge her the storage fee.
Bottom line, it is not a cheap situation you are in, regardless of what you do. You can keep it cheaper by not hiring an attorney. The ATTORNEY GENERAL'S OFFICE has some very good resources on landlord-tenant issues, which this isn't quite, but they may assist you with ideas for lease modification. Try to get your landlord on board. Maybe they will serve an eviction notice on her, essentially, for some kind of breach they view she made with the lease agreement.
Good luck. And next time, do background checks on who you room with. The best indication of future conduct is past conduct, and I would bet you she's done this before, or something like it.