This feature is better done in MyBB than vBulletin (sorry I keep hyping on about MyBB), I don't even use it anymore. But there's a setting you can use to "auto" trim users away based on posts count. It works for both members and validating users and you can choose how many days after first registering the cron job is run. I had it to run every 7 days to remove people with a post count less than 1 (0).
I used it all the time and thought it was a brilliant feature.
You could get on with running the forum knowing every 7 days members with a 0 post count got removed auto when cron job ran. or how often you wanted it to run, 30 days e.t.c