There is NO delete box for that sitemap. I can ONLY delete sitemaps listed under "mine".
Simply login to webmaster tools > Site configuration > Sitemaps > on the right of page it says: Show submissions: Mine (0) - All (1)
For all my other websites I own it says Mine (1) and i can click checkbox and selected delete there.
This is the first time in years I've had this issue.
Since we are going to "you obviously just don't..."
Here's one of my sites: http://pinstack.com
I've been doing this for over 8 years and have web properties totaling over 10M monthly page views. This script is clumsy to say the least and documentation is beyond poor. I can't wait to get it from under my skin and out of my webmaster tools.
I've used vbseo sitemap generator for the last 3 or 4 years and all their sitemaps are listed under "Mine", thus I can easily delete, etc.
So yes someone "obviously just don't understand how it's supposed to work"
Yes it will purge but this is just poor it should have been done RIGHT the first time like all the other sitemap add-ons for vb, worpress, joomla etc that I use.