The issue with re offending users is likely because a limitation to SMF. Moderators can't see if a user has been reported or had x amount of posts deleted recently, unless they manually check via the modlog. On a very busy day of handling reports, a moderator might see several hundred, and at its peak of reporting probably thousands of reports a day, and therefore potentially thousands of users. Now, because of the way reporting, and the report queue works, multiple moderators might be dealing with those reports at the same time, and therefore that's where the consistency might be lost.
That's why I think a regular of the board who can proactively keep it clean with some help from user reports would be better than having global mods taking care of mod-less boards on a 100% reactive report basis. Someone who is actually hanging out there would know immediately who's truly disruptive and who's just occasionally trolling.
And all this talk about numbers... it's not really a math thing. Someone can have 100 posts deleted out of 1000 and still be a valuable member. Shit, someone could have 15 out of their 20 first posts deleted if they didn't know some obscure rule - which by the way is not an easy thing for a newbie to figure out but that's a whole different discussion - and they could still be a valuable member, just may need a little nudge in the right direction. Mod discretion FTW.
What gets my goat is persistent willful spammers and shitposters, who take
enormous numbers of reports to get even a temp ban.