We are using a Google form for that:https://tinyurl.com/dominion-abuse
The reports can only be seen by a few mods, and it helps keeping all the information in one place. You need to enter a few pieces of information (like involved players and a game ID), then describe what happeneded (like you did here).
We discourage players posting the reports publicly (like here), because in the past there were a few cases of backlash from the reported on players to the players that made the reports.
Please use the above link to report the incident with more detail (like who your opponent was).