How to stop minification of js files in grunt?

Look in the Gruntfile. There is probably a reference to a task named "min". Try to remove that reference and run grunt again.

For a more detailed answer you need to add the Gruntfile to the question.

Edit: It could also be called uglify. Look for a line like this at the end:

grunt.registerTask('default', ['jshint', 'qunit', 'concat', 'uglify']);

and remove the last option. You might also have to change some file paths.