Is it possible to make mod which will do next?
If members number of post per day drops below 0.3 (or similar) that automatically change members group and put that exact member into group of inactive members - if number rise above 0.3 – it will back that specific member into normal groups – which means post count groups.
If similar stuff exists I will appreciate tip for it
This feature can be used also with rules – One rule will be above mentioned, another one will be for members which post 10 messages per day etc. I think people will love it and it will surely find purpose on lots of SMF forums
Spel
This is very close to this problem - http://www.simplemachines.org/community/index.php?topic=213111.0 - Who read both topics will know about what I am talking