OK, I think I understand now.
First thing you need to do is back up your existing sites, especially if you want to retain any of the copy (written content) on your new site. If that's all you want to keep of the old site, you could just copy and paste that content from the site in your browser to a word file and then use the Word Import button in the "add new post" interface to import that content into the new site.
After you've backed up and gotten what you need from the old site, you'll basically have to erase the contents of the directory for that domain. Now, this is a HUGE deal, and once it's gone it's gone for good. (Basically, for all intents and purposes.) So PLEASE make sure you've backed up, downloaded the back up files, and made a copy of all the written content you want to port over to the new site BEFORE you delete anything.
If your hosting company uses CPanel, you can use File Manager for the deleting task (there may be a better way to do this, maybe others know). Just navigate to the root directory for the domain, highlight/select the files you want to delete, and click delete.
Then you'd do a clean install of WordPress (if your host company has CPanel, you can use the Fantastico script which will install 3.2.1, then once it's up and you can access the dashboard, you can upgrade to 3.3).
From there, I'm assuming you know what to do since you've already set up one site with WP.
As to the "clear the dashboard" thing - actually, you'd have a completely separate dashboard for each site. It's not like wordpress.com, where you can have multiple blogs all accessible from the one dashboard. So if you set up WP properly on each separate domain, that would take care of itself.
Does that help?
Please do wait for others to comment on the whole "deleting what's there" issue before you go pulling the plug on your existing sites -- there could be issues with this I'm not aware of.