I remember years ago when I ushered a mass by Pope John Paul II in New Orleans. One person (perhaps more) went up as a Eucharistic Minister and tried to sneak out with an entire chalice full of hosts – SICK!!! A priest in a former parish told me that, as a Eucharistic Minister, if I saw someone walking away without consuming the host, I was to stop my Eucharistic activity and pursue that person. There have only been 2 times when I had to say something to someone. I try to do it discretly, but once it became obvious to the congregation that something was going on. After mass, the priest said that I did the right thing.