This was a question I got today at my firstname.lastname@example.org inbox and before I answered yes or no I emailed back: Did you just get in a fight or do you no longer want to be friends with her? Because firing a bridesmaid is pretty much saying I don't want to be your friend anymore.
The bride replied that she doesn't want to be her friend. She only asked her to be her bridesmaid because the bride was her bridesmaid a few years ago and they have since drifted apart. Also, the bridesmaid had done a few things lately to piss her off.
So, I replied that you can simply say that you think you're drifting apart and that you are upset with the few things she has done recently - and name them - and that you don't want her to be a bridesmaid anymore. In fact, because being a bridesmaid is expensive you're actually doing her a favor!
When you and a friend are drifting apart and there's very little connecting you anymore it's okay to let go and say goodbye. It's kind of like a break up where you both know it's over but neither of you have the balls to say it.
In this situation you just have to have the courage to say, "You're fired." But in a nicer way. You're not Donald Trump and life is not a TV show.