How to quit an agency?

I have been working for them for several months. The only place they send me is a women's prison which I absolutely hate. I am suppose to work tonight and am scheduled the rest of the month 1 day a week per my availability I turned in a month or so ago. I applied for hospice and asked about mens prisons and other areas but I feel they are so money drive by each office they refuse to give me info and shrug my questions off.

I can not take it another night, I dread the thought of having to go back to this place. It's not that it's that bad as a job per say, but several reasons why I dread it:

Being locked in a "Pill Room" for 12 hours. You finish your work in a matter of 2 hours, sit there for 8 hours, pass pills for an hour then wait for the next shift.

Unairconditioned 99% of the time. This is a killer for me. Sitting there sweating my ass off, trying to find relief.

I am a male nurse and as such do not feel it appropriate nor safe to work the infirmory/ER or any area where I am in direct contact with the female inmates. They (Jailers) do a great job ensuring we are never alone, but still inherent danger of accusations flying and unneeded stress.

I am used to a busy medical/surgical floor at a hospital. As I said I am scheduled to work tonight, and I truely feel bad for doing this but I need to know how I should go about telling them I quit. I think I want to leave this agency as a whole as they have not been doing a good job at staffing us lately (saying we are confirmed for a shift then getting to the site and being told we are not on schedule happening a LOT lately).

Any info or knowledge greatly appreciated, thank you.