The following query should delete all users who registered more than a month back, and still haven't posted anything. They may either have never clicked on the e-mail link, enetered a false email, or never bothered to post.

delete from user where posts = 0 and joindate < 960967002;

It'd probably be wise to do a select statement first, to see how many users you'd be deleting.

select count(userid) as count from user where posts = 0 and joindate < 960967002;

And then, to see all the records.

select * from user where posts = 0 and joindate < 960967002;

If everything looks good, then you can execute the delete command.