In the main view press "D" (Shift + d) At the bottom it asks "Delete messages matching:"
Enter the following and press enter.
All messages will be marked for deletion. Now just press "q" to quit and "y" to confirm you want to delete all those messages.
(The ~s makes it search the subject for the given expression and of course ".*" is a global regex wild card.
See this page for more details. http://www.mutt.org/doc/manual/manual-4.html)