I know that is a small sample but it is still impressive.
Should the role be nerfed a little or are we supposed to somehow block the player ?
I think hampering the Mercator might work a little in a real life game but the options to do so are limited in this game and such ganging just doesn't happen on BGA unless you play with a regular group. Basically, you can try to rely less on the market and more on the favors (in a 4p game) and cities and possibly get the best contracts even when you don't really intend to fulfill them. But this remains limited and can very quickly cost you too much points.
So, how could the role be nerfed ?
Here are a few ideas:
1) the power only works when at least one of the dice used for the action is colored.
2) or the power at the market only works once per round for each category.
3) or when receiving gold or silk, you must pay one gold or camel or merchandise or VP (to the bank) to get it.
Anyone with access to the game database could run some stats on how often the different personas win the in a 4 player game (as a % of the games where they are present) ?