Is it ever appropriate to intervene when someone “unworthy” is going to take the sacrament? What level of “unworthiness” makes such intervention justified?
There was a big disagreement about this on my mission. An investigator who had been given a chapel card showed up for church just as the sacrament service was beginning. When the bread tray came to him, he was unsure what to do, so he took a piece of the bread. Just then, an older missionary who was serving in a branch leadership role jumped up from his seat on the stand and commanded the man (in Spanish) to “Drop it!” which he did. 
Needless to say, the investigator never returned. To explain his actions, the missionary referred to the following scriptures:
3 Ne. 18: 28-29 28 And now behold, this is the commandment which I give unto you, that ye shall not suffer any one knowingly to apartake of my flesh and blood bunworthily, when ye shall minister it;
29 For whoso eateth and drinketh my flesh and ablood bunworthily eateth and drinketh damnation to his soul; therefore if ye know that a man is unworthy to eat and drink of my flesh and blood ye shall forbid him.
While this missionary was perhaps alone in his strict interpretation of these scriptures in this specific situation, there is clearly a scriptural prohibition to prevent someone taking the sacrament unworthily.
Should non-members be asked not to partake? The current mormon.org provides a description of our services for visitors:
A typical meeting would consist of the following:
Hymns: Religious songs sung by the congregation (hymn books are provided).
Prayers: Offered by local Church members.
Partaking of the sacrament (communion):The sacrament consists of prepared bread and water, which is blessed and passed to members of the congregation by priesthood holders .
Speakers: Typically a meeting will have two or three assigned speakers.
We do not pass a plate to request donations as part of our worship services.
There is no specific prohibition made here instructing visitors to not partake. Does that omission imply non-members are welcome to partake in the spirit of worship although it is not a renewal of their baptismal covenants?
Children and the mentally disabled frequently partake because they are considered innocent and in no way unworthy. What about ex-Mormons who are not returning to the church but attending due to familial obligation? What about crazy people (there are lots of brands of crazy)? What about ex-Mormons who are crazy?
If intervention is desired, who should intervene? How and when should this take place? Isn’t a lay member intervening kind of like a citizen’s arrest? This is one area where I agree with Elizabeth I: I have “no desire to make windows into men’s souls.” Nor have I seen anyone burst into flames for touching the sacrament or keel over dead like Uzzah the ark-steadier. My tendency would be to leave it up to the individual, but perhaps I am being remiss.
And when do you feel you should recuse yourself from taking the sacrament? For major infractions only? If you are yelling at the kids to hurry in the morning?
Wheat & Tares 