if not, you can obviously delete them from Manage | Computers.
Also, it might be your drill-down isn't configured and thus you can't delete, but...
OH, and if you really wanted to get fancy, you could use I think it's called Deleteitem.cs or something which is out there. There's documentation on how to use this somewhere in the KB - I just remembered it while typing this. It allows you to put in a SQL query for GUIDS and then goes through and deletes all the associated GUIDS. WARNING: if you mess this up, you can seriously damage your NS - think about it. Removing the wrong GUIDS could cause a HEAP of trouble. But if your list is huge, and you're 100% sure about your query, it's an option.
JLawson has the safest of all answers, but I think the report should be able to work. If you can't, maybe that's something I'll work on. :D Might be fun!