I'm angry that my girlfriend unfollowed her ex on my Instagram.
Are you mad that she used your account?
She would follow someone with it, unfollow them and block them
Significant others should not use ones social media accounts without permission, so I understand your anger, as it's a violation of trust. You should speak to your girlfriend and set boundaries. Let her know that such actions are not acceptable.
I'll just cold heartedly delete all her pictures.
To delete her pictures you would have to be using her account. Isn't that as bad as what she did to you?