Update here

Background info:
Started communicating with a company to do PSD to theme conversion. They sent me the layout, we discussed it over the phone and in emails and agreed upon a set number of hours. We signed an agreement which specifically stated hours of operation, fees for working outside those hours, anything outside the original agreement would be billed accordingly and they do not own or have rights to use any work I've done until I've been paid in full. We also did not include a deadline. I get a deposit of 50% of the original agreement. This was about a month ago.

At the start of the project:
They requested the work to be started on a Monday and be completed on Friday. I told them I had other clients but I would do what I could. Then I get the actual designs which are obviously more than we had discussed. I make this observation known multiple times.

During (11 business days total)
Client requests immediate response to emails, calls and chat messages even though they do not respond until well after my business hours much of the time. For example, on Friday I sent out a list of questions for them to answer and I do not hear back until late (11pm) Sunday night with a request to respond by 8am the next day. Monday I wake up to multiple emails and chat messages (before my operating hours) asking if I'm working and when I'll have the project completed. This has happened multiple times.

Multiple additional changes have also been requested putting the project at about 125% more than originally estimated. The additional changes are functionality based which we had discussed would already be done by their team at the start of the project. I make these observations known.

Final straw
I tell the client that I can no longer work under these circumstances and they're taking valuable time from my existing (read: paying) clients. I tell them that our agreement did not include a deadline and that I was not on call. I also did not appreciate the communications outside my operating hours nor the numerous changes and demands beyond our agreement. This was after an email I received at 9pm requesting, yet again, a response before 8am the following day because their delivery date for the client was the 1st and I was supposed to have all of this completed before then. Which is not what I agreed upon.

He says I'm being incredibly unprofessional and my invoice was out of line. I reminded him of the terms in the agreement we both signed and the numerous things he's done that I would consider unprofessional. He refuses to pay the invoice stating that the work is not completed. I again remind him that my end of the agreement was completed at the end of the set number of hours we agreed upon.

I just checked the site and he's using my work without my permission. I sent him an email simply stating that I saw he's using my work and that he had no right to until I was paid for my work and then copied the section of the agreement that stated that. No response.

TL;DR
Client signs agreement without deadline. Turns out they wanted much more than original agreement. Work goes more than double the original estimated time. I tell them I no longer wish to work with them and send them the invoice. They refuse to pay even the original agreement balance and are now using my work without my permission.

Options?
They are located a few hours away from me in another state. I've documented everything I can. I don't want to get a lawyer involved but I will based on principal. Is it worth it or should I file with their local small claims? Is there anything else I can do to persuade them?