How to hard-Delete Objects Using the Salesforce Force.com Migration tool

For Sandbox and Dev orgs only: does not work on production orgs.
use purgeOnDelete="true" in your build.xml to permanently delete your destructive changes.

For Prod:
You can download a modified ant-salesforce.jar here from the FinancialForce.com Developers Github repo (be warned this is API v22.0, though you can use it to deploy code at any version).

Or
If you want rebuild a new version, then you need to download this Java class, compile it, unzip the ant-salesforce.jar (rename .jar to .zip), put it in and zip it back up again and your all set!