How to prevent the addition of HTML in "Full HTML" text format?
Ok, I think I've found the answer: 

This is a "feature" of the Full HTML mode.


  Full HTML:
  
  
  Web page addresses and e-mail addresses turn into links automatically.
  Lines and paragraphs are automatically recognized. The  line break,  paragraph and  close paragraph tags are inserted
  automatically. If paragraphs are not recognized simply add a couple
  blank lines.
  


You can disable it in You are here Administration » Configuration » Content authoring » Text formats by unchecking Convert line breaks into HTML (i.e. &lt;br&gt; and &lt;p&gt;).

Source: http://drupal.org/node/503754