You can to a certain degree, yeah. There was a project in the UK that looked at using NLP to determine what age/gender child groomers are (to see if they were adults impersonating children in order to befriend children online). It was pretty successful (the Isis project - see http://www.scientificamerican.com/article.cfm?id=pedophile-p... for an overview). The problem is that doing this sort of thing "live" is ethically dubious - not only for the privacy of the child but also for the privacy of the potential offender (because being labelled paedophile is probably one of the worst things you can be labelled - imagine if you are falsely accused).
So yeah, you'd need a huge amount of certainty that this was "the guy", and, frankly, storing enough data about individual writing styles for comparison means it's highly unlikely (right now) that this would be possible to do from a technical perspective, and potentially ethically dubious even if the technology were there - what constitutes "public data" on the internet from a privacy and data protection perspective?
So yeah, you'd need a huge amount of certainty that this was "the guy", and, frankly, storing enough data about individual writing styles for comparison means it's highly unlikely (right now) that this would be possible to do from a technical perspective, and potentially ethically dubious even if the technology were there - what constitutes "public data" on the internet from a privacy and data protection perspective?