How to disable Maven auto import automatically in IntelliJ IDEA?

Uncheck Project Settings | Maven | Importing | Import Maven projects automatically.

For IDEA 14+ it’s under Settings | Build, Execution, Deployment | Build Tools | Maven | Importing | Import Maven projects automatically.

For IDEA 2020+, open Settings | Build, Execution, Deployment | Build Tools and check "Reload projects after changes in the build scripts" and "Any changes"