Imho, you need to manage the "freeze stats" of vbulletin script, before think to add any automated banning system.
Let me try to explain what i mean.
When you become banned, your profile, rep points, post count, and all the rest, become stored and "freezed" (in the sense that, if the admin don't change them manually, all these data cannot change for all the time you're "on vacation"
)
But this means also that, if you just add a command for ban a person when he reach -50 neg rep points, as example, he become automatically banned forever, cause, when the ban end, the script check again, still see the same amount of neg rep, and re-ban you on-the-fly, and so again and again endless.
Other than this, the rep points, if i recall correctly, depends both from the number of neg votes, AND ALSO from the rep points of the peoples that give those neg votes, so, you can go at -50 from 25 or 30 "normal" users, with standard neg rep, or from 4 or 5 "high rate" users, with high rep points (i'm not completely sure about the cyphers, you need to ask c0ld about how he set up the script, but i at least know that VB script have this feature, in it) ..... so, you have to decide how much negative points you need for an automated ban both considering an average amount (or, maybe, only the number of the peoples that gave neg reps in a certain amount of time, as example, if 25 peoples give you neg rep in a year, this can have any reason, if the same number of peoples give you neg reps in a pair of weeks, means usually that you've really done something wrong), AND also modify the script for change automatically the neg rep points of the banned user before the end of the ban, as example, putting it back to zero, for avoid the endless loop.
It's just a personal idea, but i think that, basically, can be safely done, if you take care of these details ..... and, ofcourse, also a banned user must have the right to ask for the admin intervent, if he/she think that for some reason was a "conspiration", or a misunderstanding, so the admin still have the last word (but this is normal
)
Also, what about a "warning trigger" function ? ..... as example, taking the -50 as ban limit, you get an automated PM at, like, -25, that say you "be careful, you're near to be kicked out, if you don't change your conduct" ..... or something similar