1) a goodwill email, an up-sell to a longer-term plan or extra features, etc;
2) automatically cancel these folks, and send an email to that effect: "We notice you're not using X, so we won't be charging you at the end of your trial." They will just cause a cancellation anyway, though you will lose some of the 'free money' from lazy or unaware users.
3) Don't bill, but you could automatically extend the trial since they haven't used it, and you'll get more value from people converting to group 1) than 2). Perhaps the free trial starts when they've actually done some impactful action other than create the account.
There's way more segmenting you could do based on the types of users you see, which ones are most enjoying the service, and so on. Just seems odd these days to say "I'll email everyone" on a fixed rule.