Rules of good manners and respect for the opponent.
If there are unfinished games, before starting a new one, a player must offer their partner in the unfinished game(s) to finish it. The offer should be made in a messenger for quick communication (ICQ/Skype/MSN). And only in case of refusal (or no response within 10 minutes) should a new game be started.
If a player starts a new game without making such an offer, their opponent has the right to immediately submit a report of their victory.
The reverse situation: if within ten days a player has not received any message/response/refusal to finish the unfinished game from their opponent, they have the right to immediately submit a report of their victory. Regardless of the opponent's "valid reasons."
In both cases, the player has the right to submit a report, but is not obliged to do so. The player makes the decision based on the situation.