Hi,
Open Messages.
Go to the Preferences from the Messages app menu
Go to Accounts
Sign Out.
Also Untick the "Enable this Account " box so the Account is not On.
Many people report that they never set up Messages for iMessages but then report the app keeps asking for the Password.
You need to make sure the Account is Off and Signed Out

9:23 pm Wednesday; October 3, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
MacBookPro 15" 2016 (High Sierra 10.13.x)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)