totof,
There's not a hack or plugin for this that I know of. Browsing through the blogs and looking at the date of the most recent post would be one way to do this, but perhaps you have too many blogs to do that. You could use a mysql query in phpmyadmin to track down the inactive users. What version of b2evolution are you using?

Thanks for your help personman.
I'm using b2evo 1.6
I have not too many blogs so I can go through all to find empty blogs.
I thought writing a mysql query to find them, but before I wanted to be sure that none had written it before.

Regards

totof