Sign out of messages on Mac (iMessage) I logged into my iMessage account on a friend's Mac but now I can't figure out how to sign out.

How can I secure my messages from appearing on that computer?
You can sign out of the Messages app on OS X as follows:


https://support.apple.com/kb/PH22359


If you can't access the Mac, then changing your iCloud password or evoking the application specific password for FaceTime and messages would suffice but also be more work for you. 


https://support.apple.com/en-us/HT204397
https://appleid.apple.com