Named for the boundary around a black hole from which there is no escape once crossed, this trope uses the black hole as a metaphor for evil; the Moral Event Horizon refers to the first evil deed to prove a particular character to be irredeemably evil. Note the word irredeemably. This is distinguished from Kick The Dog not so much by the extreme of evil deed (though obviously the severity of said deed is relevant) as by its demonstration of permanent evil; as in, the first evil deed whose role in the story is to tell us they will always be as evil as they were upon crossing this, or worse. Note that this does not mean their morality is always decreasing after crossing this; a character can cross this, then become much more horrible than they were when they crossed it, then become just a little better than they were after THAT, but definitely no better than they were upon crossing the Moral Event Horizon; otherwise, it was not a Moral Event Horizon in the first place.
And while I wouldn't necessarily characterize anything going on in the Foobiverse as "evil", I can easily take this concept and apply it to moments where characters became irredeemable; that is, the point after which they can't ever come back and be truly sympathetic again, due to their character being intrinsically linked with their earlier bad behavior. For example:
Deanna may have hit this point when she arranged to have a fake wedding and deceived her parents, or, failing that, where she oops'd Mike on their honeymoon.
Liz may have hit this point with the "WAIT" strip, followed by her sudden bout of homesickness mandating her move back to her parents'.
Mike may have hit this with the fake wedding above, or, possibly, even earlier with his reaction to Deanna's car accident.
Anthony, of course, has "Wait for me! Promise me you'll wait!" although it may have come even earlier.
So where's your point of no return with these characters?