A plugin to remove all external links in posts ?

Can’t think of one offhand, but if you know the links you want to remove, you might try something like the Search & Replace plugin to find the text in your database and replace it with blank text. It’s worked pretty well for me whenever I had something I needed removed from a large number of posts.

Backup your database first.

http://wordpress.org/extend/plugins/search-and-replace/