Since the SSI you are referring to is a program administered by the Social Security Administration (SSA) then I guess it doesn't actually have an office. However, sometimes Google is your friend and you get the right information
Survey Sampling International:
SSI is Survey Sampling International, a survey company and has nothing to do with the SSI program by the Social Security Administration, nor does it have anything to do with the Administration itself.. Opt out: https://www.surveysampling.com/contact/
I don’t personally know if the above link will actually take you off their list, but I have been told a couple of times that it does work.