How to get rid of old unwanted mail?
You can of course just mark the mail you don't need anymore by saying ‘#’ with point over the mail and then say ‘B DEL’ to get rid of them forever. You could also instead of actually deleting them, send them to a junk-group by saying ‘B m nnml:trash-bin’ which you clear from time to time, but both are not the intended way in Gnus.
In Gnus, we let mail expire like news expires on a news server. That means you tell Gnus the message is expirable (you tell Gnus "I don't need this mail anymore") by saying ‘E’ with point over the mail in summary buffer. Now when you leave the group, Gnus looks at all messages which you marked as expirable before and if they are old enough (default is older than a week) they are deleted.