I'm going to tell my roommate she can't get a new dog.
What happened to your roommates dog?
It died.
Although you guys share a residence, it's not appropriate to tell your roommate what they can or can't do. Have a talk with them about it.
I won't talk to her. I just don't want her to get a dog at all!
That is unfair to your roommate. You shouldn't tell your roommate what she can do. If you don't like dogs then maybe you can communicate that to her instead.