disclaimer: I personally don't see any need to remove the communication client, it is tiny and does not 'bloat' Opera in any tangible way (contrary to uninformed opinion), and you may even start using and liking it. People seem to have become, somewhat justifiably, over-sensitive to 'bloat'. I absolutely agree that software that includes too many functions overreaches it's purpose. M2 is a ~200kb client that is dynamically loaded - it does not enter that league. If you don't set up a mail account, then Opera doesn't even load that mail client, causing no bloat whatsoever. However user choice is paramount, and because of Opera’s great flexibility, it is very easy to disengage the mail client from the browser. Here is the information to do it.
Note that these instructions below applies to V7.23 and earlier, see the bottom of this page for V7.5x notes.
If you haven't yet started on the road to customising Opera, you may just learn some valuble general techniques along the way too, see EditingINIFiles for more technical information on Opera's extensive customisability.
Make sure you have set: Preferences » E-Mail to 'Use default email client on computer' before you start...
Go to [Preferences > Programs and paths] and uncheck "Enable mail and chat".