I cancelled my membership a year and a half ago.
I said, "I would like to cancel my membership. The green party's stance on transgender issues is not something I support and do not want my name to be a part of it."
It looks like I deleted the email I received in reply, but it was something along the lines of "sorry to see you go, we'll forward your concerns on to the relevant people blah blah blah"
I was only a paid up member for over a year, and am not particularly political in the first place, so I wasn't overly upset to cancel.