Admin Accounts Making Themselves Site Owners

Hi everyone,

I've made a SharePoint site from scratch and when I logged on the other day to look at permissions, I found that an ICT admin had made themselves an owner of the site. I don't know why as it's nothing to do with IT - they have never been involved in the site I did it all from scratch myself. I've now kicked them out but they still have access to a load of documents as an owner. What I want to know is:

1) How do I stop people adding themselves as owner?

2) How do I stop them having access to the documents?

Any help gratefully received.

Thanks.