Hi Adam,
Thank you for your quick reply. 
I agree on a policy which lets some place for errors, especially for younger players as you said.
Several points caught my attention:
- Between the scammed game, the signaling, the annulation and his/her second acceptance of my public challenge only five hours had passed.
POV
Imo a short temporary ban would be justified, to get the message across that cheating is not tolerated, and to let cheaters (hopefully) reflect on their behavior before they resume playing on OGS.
- The mod detected and annulled several other scammed games. Before I signaled my encounter with him/her I went myself with AI over other games by the same player - I am reasonably confident that there are more occurrences of cheating than flagged by the mod.
POV
Going through someone’s games takes a considerable time if done manually. I wonder if the cheater could be brought to take some responsibility for this. For ex. that he/she could be obliged to auto-signal the games in which he/she cheated, or else get the result of all of his/her games annulled or be put on a longer temporary ban? This leads to the next point:
- The players rank doesn’t reflect his/her strength if there were scams before but the other games I had with him/her before weren’t annulled.
POV
This could distort my as well as his/her rank, and everybody else’s. As far as I can see a new player finds relatively fast a rank reflecting his/her strength. In regards of that and the second point it might be reasonable not to bother with the scammed games at all and just to reboot the cheaters rank? Or would this impact the overall rank distribution on OGS to much?
I wouldn’t like to see the cheater banned if this was the first time he/she did it - but I certainly want him to be held in some way responsible for his actions, besides the annulation of some of his/her games.
This answered my question, I seldomly play unranked games - I forgot that they are shown like this. ^^
Thank you again for your quick reply.