How to disable the target for application extension when building in xcode?

In order to disable building/running the NSExtension you build, you should:

1. Click the project file in the project navigator
2. Click the containing app target (the one you DO want to run)
3. Click Build Phases tab
4. Open Target Dependencies or Embed App Extensions
5. Remove the extension (the one you DON'T want to run)

To bring it back, simply click the + sign in the same place and re-add it.

In Xcode 9 the Extension was stil visible in the Widget List of the phone. In order to hide totally it I had to remove it from "Embed App Extensions". The procedure is similar to the one described by nurxyz:

1. Click the project file in the project navigator
2. Click the containing app target (the one you DO want to run)
3. Click Build Phases tab
4. Open Embed App Extensions
5. Remove the extension (the one you DON'T want to run)