FemmeLegion
Maybe it's because Florimel has an Ascendant account, but I was able to click into some of his campaigns to which I had been invited and remove myself. (He wanted me to play but it was on days I couldn't make it.)
From the main page, go in to "Manage Members". My own name had the option next to it to "Remove Player". Naturally, I did not have this option for any other players.
Anyone able to do this as a player in a non-Ascendant campaign?