Banning accounts doesn't though.

Just removes the potential for future contributions (and gauging the 'value' of the contributions I've seen thus far... well, I'm sure you're a better judge than I).
On that note but veering slightly more off-topic...
You might consider updating your forum software - I think from vB 3.6.7 onwards, they introduced the '
Infractions' system.
Customisable and cumulative, you assign a value for various types of offences and under totting up, can lead to either a temporary or permanent ban.
I use it on my own forum and it's great for dealing with spammers too - just assign a 100 point limit to a permanent ban and a 30 - 50 point one for a temp ban.
10 points for minor offences (like trolling), 30 - 50 points for more serious offences, such as posting pr0n or warez and 100 points for one-hit wonder advertising spammers.
Works great - can be configured to automatically create a thread in the admin/mods section and send a message to the user in question as to why they've received the points.
PM me if you want any further advice/info on this kind of thing. I've been running my own vB forum for quite some years now.