Technically, ICANN is a California not-for-profit corporation so it is primarily accountable to its corporate board of directors.  Under the law, the buck stops with them because they have a fiduciary obligation to make informed decisions that serve the public interest.   If they fail, one could appeal to the California Attorney General's Office who over-sees California nonprofits.  One could also complain to the US Federal government because of ICANN's 501(c)(3) tax status it must be meet certain standards of accountability and public benefit.   And ICANN can be sued in legal courts, most easily in California, just like any other nonprofit corporation for breach of its legal obligations.

Robin