In the main view press "D" (Shift + d) At the bottom it asks "Delete messages matching:"

Enter the following and press enter.

~s .*

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)