I'm sorry, that's not possible. See http://www.wikitree.com/wiki/GEDCOM_FAQ
Since WikiTree is a collaborative environment, it's not the same as managing your own tree on your desktop. If it were only your tree, you could delete, replace, whatever. But here, we're all working on one tree. If you delete profiles you could be deleting what other people are seeing and editing and connecting with their own families.