Just email them and give them 1 week deadline to response to it.
Why not do that?, almost everyone opens there email.
Its there fault if they are not opening there email.
If you dont care about the one who holds the data, Then why bother to delete them
But If you care then delete them since they store a lot of info (Since I am a Web Admin also who back up site datas)
I think for those who dont post just delete them because they can still freely visit the site even without Account.
Your right...
Just once a month\week open there account thats ok for me but not those person who opens there account once a year or even never.