sorry, but I am not aware of an option to limit this.

I don't know of a solution to prevent multiple login's using the same account; But you could create an account for each person who will have admin rights, then give them the administrator role. This will possibly allow some tracking, as well as being able to see which other admins are online (using the users online module).

Once DNN 5 comes out (me wante) it will be much easier as the Admin/Host menu's will be 'normal' pages and other users/roles could be assigned to them, ect.

A solution to the multiple login's on a single account you are having will depend on why that is a problem - If it's to limit each admin level account to 1 person, my first suggestion should cover it. If it's to stop people from editing the same content at the same time, I'd use my first suggestion and have everyone use an IM, at the very least when they are going to edit content, so they can check who else is online and make sure no one else is trying to edit it.

If there is another reason for you wanting to block multiple login's, let us know what it is, and we might be able to find a sufficent solution for it for you. :)