People can't use keyloggers to hack the server through in-game computers.
In-game can only run lua/forth and they take input from in-game interaction of computer.
Sure players could download keyloggers with computercraft computers but only if the http api is enabled.
Besides players can only run programs supported by the in-game computers' operating systems.
And the in-game operating systems DO NOT have access to ANY information from the server outside of normal user interaction in-game.
Which means it is completely POINTLESS to ban them.
But it's only pointless to ban them for the reason YOU STATED as why you banned them.
So either find some other reason to ban them or unban them.