Guests who invite themselves

What do you tell people who invite themselves to your baby shower when you have no intention of inviting them?
My husband's coworker's girlfriend says that she bought the baby stuff and she keeps telling my husband to let her know when the baby shower is. My husband is close with his coworker, but not the girlfriend. I've only met her once, and we are doing a traditional women only baby shower. What would you tell this woman?