I got asked to following question, which got me thinking what's the policy in different situations:
On a Game Day event, Anton controls 2 Knight of Glory. Natan kills one of them with an Orzhov Charm, and the other one with a Doom Blade. No one calls a judge.
After another round a player comes over and tells you he heard Natan bragging that he knew what he did was illegal, and he did it because he knew Anton wouldn't notice.
How do you handle the situation at that point?
What if this was a casual event instead of regular? How do you deal with cheating then?
What if this was Competative? Would you give Anton a Warning for FtMGS?

Regardless of REL, it sounds like Natan has cheated. You would have to investigate (speak with Anton, Natan, the player who told you, as well as whoever Natan allegedly bragged to).

Based on the investigation, if Natan did indeed knowingly perform these actions, with the intent to gain advantage, you disqualify him.

Regardless, I wouldn't give Anton a Warning for FtMGS at Competitive REL. If you've ruled out Cheating, then the match is over and done with. And we don't tend to give out penalties like that after the match is over.

I agree with Mark. Cheating is cheating, regardless of the REL. By the
Player's own admission, they preformed an illegal action because they
“thought they could get away with it”.
You'd have to investigate, of course, but assuming the opponent
corroborates the story, I would have no problem removing the player from
the event.
As the round is already over, I wouldn't issue any penalties to the
opponent.
Remember, you don't have to conclusively prove a player was cheating, it's
enough for you to be reasonably sure.

Thomas Edgar
L1, Sindelfingen, Germany.

JAR “Serious Problems”

Any player
engaging in the following must be removed from your event and, at the Organizer's discretion, removed from
the venue entirely:

…

Intentionally and knowingly breaking or letting an opponent break game or tournament rules, or
lying. (“Bluffing” about cards opponents can't normally see is permitted).

If the investigation confirms what you heard, it's clear that Anton knowingly broke the rules. Policy here is clear and your hands are tied; you must remove him from the event.

If the player confirms the story, that's textbook DQ for cheating. It's even cited in the JAR.

But please, be cautious. In a casual and fun environment, this could easily escalate and destroy the game experience. Be sure to make the TO collaborate with you and your investigation. Also, remind that this might be not cheating if the player wasn't aware what he was doing is illegal. Personal issues may make the supposed witness to lie to you in order to get the other player DQed.

It's always a complicated situation for us to handle. Try to do your best, and ask later here for feedback if you're unsure about that. Remind that any DQ, even at Regular REL, must be reported to Wizards, and probably your mentor or RC will be very interested in what happened there :)