If people decide they don't want see a movie, there's nothing you can do to force them to go. People make these decisions on many factors. I doubt most people will boycott movies by actors who expressed an opinion on political issues one way or the other, but even if they do, they're acting within their rights as consumers.

Ultimately, if enough people stop paying to watch movies by actors they don't like and those movies start losing money, then that could result in studios deciding not to use those actors in the future. That isn't a blacklist, it's the market in action. This is nothing new - actors fall in and out of favor for a host of reasons. Actors who make profitable movies will always be in demand regardless of their political views.

The same applies to commercial endorsements - actors who become unpopular for whatever reason are going to cut their market value. For example, a couple years ago, a lot of hunters and shooters decided to boycott K-Mart because they used Rosie O'Donnell in their advertisements and she was very vocal in favor of gun control (except for her bodyguards). If people decide to shop elsewhere, there's little you can do to change their minds.