At present I'm using your plugin with User Spam Remover.
This plugin removes users who have no posts etc to their names, after a certain length of time. The idea is that your plugin or akismet, etc, prevents the spam from being posted, but leaves the registration intact: the USR plugin will then just delete it.
Would it be possible to combine this functionality within your plugin? Why? - because I can see the value of doing an IP etc check before the deletion occurs, which will mean that logged in genuine but inactive users can still be retained (and used as contacts, membership etc). USR doesn't do that, except through using a patch alongside another login logging system, and it's much more basic than what I am suggesting.
How I would envisage this is that a check be made of users without activity after being registered for (say) between 30 and 31 days. It may be that this would involve you needing to store registration/login IPs, and I can see that might be a step too far, but, hey, you're the developer!
I can also see the value of being able to manually check part or all of the user's list against sfs etc.