How can I avoid re-encoding of video in iMovie and use it for cutting only?
Indeed, you should use QuickTime rather than iMovie: iMovie will import (and therefore usually encode) the movie, but will also force you to export, as you have noted.

The main interest of using QuickTime is to use its editing features.

Assuming you're using QuickTime X (Snow Leopard / Lion), open your movie file, and hit ⌘T (or Edit > Trim). The timeline will morph into a yellow double-ended scrollbar so that you can trim the sequence.



Adjust it to fit the sequence  you want, and validate the trimming with a click on the Trim yellow button. You can then simply save the file (⌘S), without the need for an export that will necessarily reduce quality.



Update seeing that you need to remove parts of the video.

You'll need the more advanced editing features of QuickTime 7. To install it on Snow Leopard or Lion, refer to Installing QuickTime Player 7 on Mac OS X v10.6 or Later on Apple's KB.

You'll then be able to open your video with QT7, select parts of your video with I (set input point) and O (set output point), and delete the part with a stroke of ⌫. Then, simply save the file, as said earlier.

You might need a license key for QT7, though (I don't remember if the latest versions of the OS need them or activate the Pro features for free).