I am a temporary employee. I hurt my back this past Tuesday, and was immediately sent to a clinic for examination. I was diagnosed with Lumbar Strain and given a return to work with 'light duty'. I want to quit this job, as it was only temporary anyway and I don't want to hurt myself futher. Can I just quit, and if I do, will I be liable to reimburse Workers' Comp for the initial doctor's evaluation?

Yes, you can quit - we abolished slavery in this country some time ago. You can quit any job whenever you want. :)

Your quitting will not in any way affect the company's liability for your injury. You will not have to repay them for the doctor's evaluation and if you have any further medical care with regards to this injury, they will still be liable for it even though you quit.