Remove the network drive on startup

Good morning.

I currently have a server attached to my network. I'm upgrading and will be removing the server. Currently when I start my iMac it looks for the server and asks for the password. If I click on cancel it keeps popping the password box back up (very annoying). I noticed in System Preferences under Startup Disk, that it has network startup. I'm assuming this is where the password box is coming from. How do I remove that from my Startup disk? Can't seem to drag it or anything. Any help would be appreciated.

Thanks.