Struggling to remove Contacts from Messages So I just purchased a Macbook Pro second hand and his contacts are still synced to the messages app on the bottom of the screen on the homepage. Is there any way for me remove his contacts and sync my contacts instead?
Yes it is possible.

Select the "Preferences..." item in the Messages menu. Select the accounts tab. There you should see which accounts that are connected to the Messages app. Remove the existing account(s) and the add your own account.