I mark through and write "I demand to be charged in Euros, no conversion authorized." I mark out anything else on the receipt and sign it. If it comes in any other way I charge it back. If enough of us do it, they will stop the crap. You should do it and so should everyone else so that they get tired of charge backs and stop this practice.

If it doesn't cost them more than they profit from it, then they will continue to do it.