How to sign out of Messages on iMac, High Sierra

Open System Preferences>Accounts, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

In the same pref pane highlight your old account, click the little minus icon, then if need be use Disk Utility to Secure Erase Free Space.

command-option-shift-R at startup, use Disk Utility to erase the hard drive using the View menu -> Show all discs. Pick the top most disk before selecting erase. Once erased HFS Extended Journalled, run the installer. It will be as new.

Go to the messages menu to Quit the program.

If you are perturbed by the notifications, disable them via the Notifications Manager in Apple menu -> System Preferences -> Notifications.