From the Model Mayhem FAQ:

How do I edit or delete a casting call or notice?
To alter or delete your casting call or notice, simply go to the page of your casting call or notice. All of your Casting Calls, Travel Notices, and Availability Notices are listed on your profile page so you (and others) can find them easily.

At the bottom of each of your Casting Call, Travel Notice, or Availability Notice will be buttons that allow you to delete or edit your postings.