disable iMessage on one or more devices but not all I have an iMac, iPad, a MacBook Air, and an iPhone.
I want iMessages to work only on the iPhone.  How can that be accomplished?
On the Macs, in Messages.app, go to the menu Messages > Preferences, select the Accounts tab, select your Apple ID in the column on the left (if it is there) and press the '-' button to remove it.

On the iPad, go to Settings, Messages, and turn off the iMessage switch there.