Autoaim cannot be solved absolutely by technical solutions. Even if the executable is fully electric fenced and verified matching a checksum, aimbots can just read the video buffer from a different process. In practice, you can statistically analyze player hit metrics and then monitor people who appear to be too good. However, this requires basic customer service which is less likely to come from a free server for a noncommercial game.

I don't expect it would be that hard to make an aimbot look human by throwing in some random variation. It would probably then be difficult (even for a human auditor) to reliably differentiate genuinely good players from "too good" players.

Additionally, some humans are far more skilled at aiming in a video game than you might think possible.

It may send the position the player is aiming at at certain periods, if the player is way to accurate and linear moving to the target you can ban him. Some false positives and stuff should be considered too