Removing articles from ResearchGate
I am not familiar with SSRN, but it seems that when you publish content there you give SSRN a license to display the paper, but this is not a transferable license like a Creative Commons one. If that's the case, and if you did not publish the paper on a journal or elsewhere, then you are the copyright holder and Researchgate does not have a license that allows them to publish it. Then they are infringing your copyright, and the correct legal step to take is filing a DMCA takedown request using this PDF form.

IANAL.