A quick HOWTO; from my HOWTO series.
User needed to be deleted from Site Collection.
1. Login to the site as the Site Collection Administrator
2. Go to the All People Group by visit the URL http://[your server address]/_layouts/people.aspx?MembershipGroupID=0
3. This will give you access to the All People list.
4. Click on the Name Column and Click "Show Filter Choices"
5. Select the user you want to delete
6. Do Actions > Delete Users from Site Collection