List all users whose "member for" and "last access" are the same I have around 40,000 users on my site but around 20,000 of them are automated registrations (by comment spammers presumably) and I'd like to delete those. 

It's very easy to spot them as their registration date/time is exactly the same as their last access date/time. 

How do I filter the user list in Views so that I only get users whose "member for"=="Last access"?
These won't be exact matches.  It uses a timestamp value that will vary, even if only a few seconds.

I would suggest doing this in some custom PHP

that module, or writing your own custom functions.  you'd convert the timestamp difference to a reasonable value (say, within 1 hour?) and then user_delete()

I would also add a captcha and/or email verification to help prevent this.