1) Look in:
Search the code for “moodlelogo” But this doesn't exist in Moodle 2.4
Many of the other ideas seem to want to change footer.html
2) Use Artisteer.
I create my style using Artisteer and guess what - no moodle logo at the bottom.
The Artisteer makes the coding of these style sheets very quick and this works well with Version 2.4 Moodle