Blocking spam to public Group?

Anyone have an idea how to block spam to a public group? I have a couple public groups for customers and sales leads which is on marketing materials, but I am continually hit by spam coming to our group email addresses. There are perhaps only 5-6 senders but it is 2-3 times a day and I've requested removal but I'd still like to be able to block them. Unlike other mailboxes, I see no way to mark a sender as spam. Right clicking on the group only shows a limited number of options - copy reply, forward, delete, etc. Seems like "block sender" should be a no-brainer" here. Unless I've missed something obvious in the 20 times "Groups" have changed in the last 3 or so years.