I’ve switched this topic to a Feature request as I think excluding certain categories from Trust Level progression would be a useful feature for a few different use cases, even if we do come up with an alternative for your particular case. 
It looks like it would need a change to how the user_stats are collated, so would be more than a trivial tweak, but would allow ‘secondary’ categories like student areas, Sales/Marketplace, Games, and others to not skew the stats for the ‘core content’.
(Maybe that should be ‘non-primary’ and ‘primary’ as the distinction 
)