If it turns out nobody is in there then you can hire a locksmith to get in and change the locks.I wouldn't put any utilities for the property in your name until you figure out who is in there and are able to legally get them out.  

You'll need to hire a locksmith to get the locks changed - this is assuming the property is vacant, otherwise you'll need to evict the current occupant.  

Thanks everyone for the replies,  I am just having a local locksmith company come out and ready the locks.  

Can I remove (and then reinstall) the board myself and use a locksmith to get into the doors?

When the officer and locksmith came down at 1030 am they were informed by court that my tenant filed for a stay and dropped in mailbox after court was closed Mon nite.

If tenant refuses, you show up with a locksmith and change the locks while the tenant is away, conduct your Lead Paint test, and then start the eviction.I think most of this would be resolved with more stringent background checks and higher standards.Have you spoken to the City about this to see how you would get around it?

I would fix it this time and tell her if it happens again, that she will have to hire a locksmith to replace the door lock at her expense and call it a day.

@Barry RobinholtI have been using landlord locks for 10 years... they are not cheap but I can't tell you how many thousands of dollars we save on locksmith fees.

While they were at the office signing docs, our locksmith was at the property rekeying the locks!

Knowing the location of the home, if it has a basement etc...would be helpful but snaking companies (may be the plumbers, maybe not), HVAC, general handymen/women, locksmith.