Q. 1. What is the proper response when receiving Holy Communion during the Holy Mass? Some say "Amen" while others say "Thank you" among other answers.
A. 1. The proper response after receiving Holy Communion is "Amen." No other responses are acceptable, not even a nod of the head or total silence.
If the faithful are not giving the proper response, then there is a need for the priest to educate them on the matter. The Parish Bulletin can be very useful for this purpose.
I should add, if you suspect that a non-Catholic is receiving Holy Communion by his response, you could ask him if he is a Catholic after Holy Mass. If he is not, you can then inform him that the Sacrament of the Holy Eucharist is for Catholics only.
As a general rule, during the distribution of the Holy Communion, no one can be denied the Eucharist if they approach the table to receive it.