Legal Question in Employment Law in California

Just send them a letter telling them you are no longer interested in the position or being placed by their agency. I suggest you make absolutely sure you have the other job first, though. I had a friend who gave notice at our office before his new employer had finished their checks on client conflicts, a conflict was discovered and he wound up with no job at all.

Without reading the contract you signed, it is impossible to know what the rights and obligations of the parties are. But it is difficult to imagine you would be forced to work somewhere you choose not to, as employment is presumed to be at the will of either party. If the staffing company has not incurred any expense yet, it would be better to inform them now, rather than after they spend money on a background check and drug test.