I want to share my post in Google+ with everyone except a specific user. Is it possible in Google+? (If it matters, this user in one of my circles.)
The only way currently is to create another circle with all the people you do want to share it with and to not include the person you don't want to share it with. You can then share it with this new circle and make sure you lock the post, so it doesn't get re-shared.
This is a frequently requested feature, so you should also Send feedback to Google to voice your opinion that this should be added.