how to delete orphaned "site_" groups

Hi,

every site has some system groups attached to it (site_...). In my installation, deleting a site via "My Sites" dashlet does not delete those groups, nor is it possible to delete them from admin tools.
Is this intended behaviour (I doubt it) or is it a bug (in my system maybe)?

In the log I found the following line that might be related:

ERROR [org.springframework.extensions.webscripts.AbstractRuntime][ajp-bio-8009-exec-39] Exception from executeScript - redirecting to status template error: 06220025 Script url /api/groups does not support the method DELETE

org.springframework.extensions.webscripts.WebScriptException: 06220025 Script url /api/groups does not support the method DELETE

Your help is really appreciated.

Alex