Step 1
Open a spam message (any spam will do) and click the “show details” link.

Step 2
Click “filter messages from this mailing list”.

Step 3
Go to the “Has the words” box, and change it to listid:.googlegroups.com>". This will tell gmail to match the pattern .googlegroups.com in the listid header.

It should look like this when you’re done. Click the “Next Step” button.

Step 4
Create a new label for google groups. You can call it “google groups”

Step 5
Check the “Skip the Inbox” if you don’t want to see this spam in your inbox.
Be sure that “Apply this label” is checked.
Check the “Also apply this filter…” box so that the filter retroactively works with existing messages.
Click “Create this filter” and you should see all the spam moved from your inbox to your “google groups” label.

Step 6
Here’s how to report the spam:
Navigate to your “google groups” label (there’s a link to it on your left sidebar).
Click “Select All.”
Now just make sure it’s all spam and report it.

You can also select more than one page at a time, but Gmail won’t let you report multiple pages as spam. You can, however, delete multiple pages of email.