Ninjakiwi has, anti-hack measures in order to stop hackers from joining public matches. They usually like to target the people with the most hacks because they should be stopped.
Note: these may not be the only things that can get you autobannned.
Skill Points Edit
The most original anti-hack measure. The ban is simple, having negative skill points will automatically get you banned. You may get unbanned after you respec skill points, but you usually will still stay banned.
Too many boxes Edit
Another very original one, before, getting over 13 boxes resulted in an auto ban. It is not known if this has been changed, but rumors say that it was disabled after nightmare. Nevertheless, its still risky to go beyond 13 boxes.
More info: Normally, there is a cap of 12 boxes that aren't dropped by bosses. Since you should only get one box from a boss in a non-nightmare game, getting more than 13 indicates that you've gotten multiple boss boxes, which is only possible through hacking. In nightmare, the largest amount of boxes possible is 12 non-boss boxes and 6 boss boxes, so in nm, autoban may be 18+ boxes.
Ria Trident/M1000 Edit
Before, getting one of these resulted in an autoban. As of now, that has been disabled, but who knows what happens if you do get one? Chances are you won't get banned, but just maybe. M1000/Trident before level 90 will also result in autoban.
Black Gear Edit
Black gear too soon after account creation or (most likely, but not certain) before level 25 can result in an autoban. It is unsure what exact time frame it is. Still risky too get black gear within a few days of account creation or before level 25.
More details here: When you start a new SAS 4 account, there is something called timeplayed that shows NK your time played. Getting black gear before a certain amount of "timeplayed" will result in an MP ban. It is unknown if you get instant banned or sends a check request to NK.
Level Changes More Than EXP Gained (needs more testing)Edit
If you set your level to 100 by directly setting exp when you create a character, you will be banned. For example:
1. You go ahead and make a hack that sets your exp to a high value.
2. You create account, activate hack, create character, takes you to a match, you leave match, it says that you are lvl 1 in the failed screen.
3. You go to main screen, you are lvl 100, and you are MP banned.
Failed Mission XP (needs more testing) Edit
If you get too much xp from failing a mission, you can get banned. As a way to counter the failed mission XP multiplier hack, failed mission cannot level you up more than once, and the most it can do is make you 500 xp away from the next level. If you get more than enough XP to level you up once, you are MP banned.
Spawn rate hack check Edit
Spawn rate hack can only cause autoban in MP. Since spawn rate hack is client-side only, a hacker using spawn rate hack in MP won't give extra spawns to people not using spawn rate hack. It's possible that this autoban works by checking if there are too many extra zombies for one person.