A few “write” page customization questions

Hi, I’m just starting to plan my site and configure WordPress, and I was wondering how I could change the article writing page in the following ways:

1) I want to make sure writers could only post to one category at a time; basically, change the category selectors from checkboxes to radio buttons.

2) I want to make sure only level 10 users can post to a specific category (in my case, “site announcements”).

3) I want to get rid of the trackback stuff, because I just don’t need it, and the people I’ll be working with probably won’t even know what it is.

Thanks to anyone who can help.