Mass Deleting Members

I have 20,000 free members, 50% of them have either never logged in, or have not logged in in 18 months. I would like to mass delete all of the people who have not logged in recently. How do I do this?

Bulk delete on the admin panel does not work, it is slow, and a lot of the times it returns an error. Is there an SQL command or a s2hacks php script I can run?

https://wordpress.org/plugins/s2member/