Do you think I'm wrong if I ask my girlfriend to block her friend on social media?
Why do you want your girlfriend to block her friend on social media?
She keeps messaging my friends and it's making me uncomfortable.
You should tell your girlfriend about the problems her friend is causing and suggest that she be blocked. See how she reacts and if she's willing to do it. Don't try to make her do it.
She should block her friend because that's what I want her to do.
I know your girlfriend's friend is becoming annoying to her, but you can't force her to do something you want her to do. Explain the situation that you're in and see if she understands that it's really bothering you. She may be willing to block her friend if she understands everything that's happening.