If abusive users all have IP's that fall within a range, then this range should be identified as suspect. Such suspect ranges should be suggested to the admin for banning.
Got this from one of our mods this morning, after installing the vBMail beta last night. I've duplicated the issue, test banning a user from the AdminCP.
Not sure if it matters, but I also run vBOptimise (with MaxCDN) and DBSEO, along with about 30 other plugins, with all-https and a frontend...