The slam jumping very easily could be labeled as an exploit because thats not how they were intended to be used. Anything used to gain an advantage in a way that was not intended that way is exploiting. This is where the loopholing rule comes into effect as the development team decides what is classes as an exploit and what is not.

Overall you seem to be confused about alot of things. One thing I forgot to point out was how to claimed the Loophole rule contradicts the idea that you can't punish for rules that are not listed. It doesn't contradict it at all and you should take careful note in what it says at the end: "An admins word on a rule is final". The key term is rule. There is a big difference between deciding what a player did falls under a preexisting rule and making up a brand new one. If you can justify a link between what they did and what ever rule it is, then by all means you can punish them. You just need to make sure to explain to the player how what they did links back to the rule. I wrote the loopholing rule to replace
Chris's idiotic common sense rule because as I stated before no player should be punished for something they would have no idea is wrong.

As for the NLR thing. The bottom line is you are the one who is going to be issuing the player the punishment. If you feel they have a valid enough reason to come back then you are not forced to issue them a punishment. This is not however the same as letting your friends break NLR just because. Its like anything on the server you must be able to provide a valid reason and as long as you can give me one should I ask about it(and as long as you aren't doing something else wrong). I would back your decision