How do I remove the Devise route to sign up?

you can do this in your model

# typical devise setup in User.rb
devise :database_authenticatable, :registerable, :recoverable, :rememberable, :trackable, :validatable

change it to:

devise :database_authenticatable, :recoverable, :rememberable, :trackable, :validatable

notice that the symbol :registerable was removed

That’s it, nothing else is required. All routes and links to registration page are magically removed too.