According to the website, the address is:
Poloniex, Inc., Wilmington, DE, USA
A more detailed address is available from this link
https://www.bloomberg.com/research/stocks/private/snapshot.asp?privcapId=309559982
Mr. Tristan D’Agosta
Founder and Chief Executive Officer
Poloniex
1013 Centre Road
Suite 403-B
Wilmington, DE 19801
There is also an address under item 23 (Arbitation) of the terms of use
https://poloniex.com/terms/
Poloniex, Inc. c/o Cooley LLP
500 Boylston St
Boston, MA 02116
Attn: Poloniex Arbitration Requests
I think a letter should be written to the Chief Executive in the first instance. Then try the second address if the Chief Executive fails to give a positive response.
I won’t write the letter, as I am not affected, but I am happy to comment and make suggestions, if someone knocks a first draft together.