Cannot update, shutdown or restart MacBook because of iTunes store

For a couple of months now, itunes store has been prompting me to sign in. However, when I enter the correct password the prompt to sign in just comes in again. Whilst the prompt is active, I cannot force quit itunes. This is preventing me from updating, shutting down or restarting my MacBook Pro as every time I try any of these, I'm promted to close itunes before it can proceed. I have tried changing my Apple ID password and signing out and back in. I'm currently running Mojave.