Do you think the case against the purchaser of the keyword is stronger if they've used your trademark in the creative of the ad? Many people use the search term as the "headline" of their ad. So if Supermarket A had bought the adword "Supermarket B" and someone entered "Supermarket B", then Supermarket A's ad would appear, headlined "Supermarket B". Surely this is passing off / mis-use of Supermarket B's trademark?