So, I seem to have split my server's staff team on this as half think that the method of gathering evidence we use is unethical, and the other half feel it's justified if we pre-warn users that it will happen if they cheat. How do we do it? We take a screenshot of the player's client if we detect a variety of hacks on their client, then send it to our web server, if it happens multiple times the system then automatically bans the player.
Some of our staff believe that we should never take a screenshot from the user's client, with or without prior warning/consent, others believe that it's okay if they violate the rules and we pre-warn them. The only reason we use these screenshots is for enforcement of the bans or to clear allegations in the case of false-positives.