Hi Nathan,
An alert set is marked as Active when there is an active C2POLICE instance in the LPAR, and this C2POLICE instance is using this set. That means, at the start of each configuration interval it reads several of the members that the set is comprised of. So deleting an active set may cause C2POLICE to abend 013-18.
If you wish to delete the active set, you just stop the local C2POLICE. Then you use D, clone from your template, make any remaining changes, Verify, Refresh, and finally restart C2POLICE.
Alternatively.... import the installation defined alert definitions from the transport file, edit your active set, select the alerts that you just imported and that you need on this LPAR, Verify and Refresh.
You could of course create a whole new set in C2PCUST, Verify and Refresh, and then edit the C2POLICE PROCLIB member so PPARM points to the new set. Then cycle C2POLICE.