In 2011, I wrote “How to Label Only Selected Features in QGIS” which ends with the wish that
Another “data defined setting” like “show this label (true/false)” would be more intuitive.
… and now we have it!
It’s called Show label and you can find it in the Rendering section of the labeling dialog.
The following screenshot shows a quick example of how to label only airports starting with A by setting the expression
"NAME" LIKE 'A%'
This post was motivated by a question by Eduardo here on this blog. Hope it helps!