First, you have every right to express your opinion, political or otherwise. But when you use your fame as a political platform you have an advantage over ordinary people and a platform that they don't have access to. The Celebrity Boycott List tries to be a counter-weight to that unfair advantage. My advice would be to keep your mouth shut with regard to politics, and focus on your art. Anything you say regarding politics can, and almost certainly will, alienate a portion of your fanbase at the expense of giving another portion a warm-fuzzy. I personally don't believe you are going to sway many people from one position to another. You have every right to try, but again, it will cost you. To believe otherwise is naive.
Second, to get off of the list you can publicly denounce all previous political comments and promise to stay out of politics. If you do that we will remove you from the list, but if you renege on your promise we will add you back again and you will likely never get removed at that point.
Most celebrities simply don't care about the list, or don't even know they are on a boycott list. However, some have indeed expressed concern. Those who have, so far have all been belligerent and unrepentant. So be it.