To unlink your YouTube and AdSense accounts:

1. Sign into YouTube.com.
2. Click on your username link at the top right of the screen.
3. Click 'Settings'.
4. Click on 'Manage Account'.
5. Click the link 'Change Google Accounts' under the 'Change which Google Account is linked to this account' area.
6. Enter in the captcha code.
7. Click the 'Unlink My Accounts' button.
8. Click 'Create a new Google Account'.
9. Enter in an existing email account and a unique password.
10. Click 'Create a Linked account'.

video walk-through on unlinking YouTube and AdSnese:

You should see this message telling you that your YouTube.com account is now linked to your place-holder/other email account.