I feel like excommunicating my cousin.
Why are you thinking of excommunicating your cousin?
I used to like him but he did something I can't forgive him for.
People make mistakes, you should be careful about cutting them off, especially if they are family. It could lead to people taking sides and fighting among family members.
I know but it's my prerogative to cut people out.
It can be really hurtful if you cut them off. I think you should talk to them about the problem you have. They ight be willing to listen and try to repair your relationship.