Not true. By the time Nexon hit 146, the client has buffstats that have the ability to raise the damage cap of the user. You would just force their buffstat to Integer.MAX_VALUE so that they have no damage cap. This is what most high-version servers do since they don't know how to memory edit past the publicly available LEN clients around 111/117.
Depending on how the redirector is setup, you should listen and disconnect their socket to the game server once they have logged off. If anything, send a unique packet to the redirector to listen to so that it can be notified and disconnect you. When you close the entire redirector, it closes your connection entirely (obviously) which is why that works. You can improve it though for sure to fix this issue.