I recently bought a iPhone 6 Plus with iOS 12.1.2 and the former users iCloud account is still active. How do remove or delete it from this device?

Ask the previous owner to follow this

https://support.apple.com/kb/PH2702?locale=en_US

You cannot. Only the previous owner who set Activation Lock can remove it. If you cannot contact them, or they will not or cannot remove the lock, the device is worthless to you. You will not be able to activate it.