@Damen Stephens: The author of this forum meant not to imply that ONLY the pronoun “who” should be used when referring to people; rather, I believe that he/she meant to imply that, between the two options of “who” versus “that”, “who” is always to be used, for it refers to people.
Although, if you wish to get technical about it (which almost always pays off), you could also argue that, in a scenario which involves a species other than humankind, it would also be correct to use “who” when speaking of the dominant species—or at least the one being discussed at the moment—, for said species would be the one considered to be the most important.
For example:
In the “Redwall” series (a fictional set of stories about mammals and other animals), since there are no humans in them, a sentence would go like the following: “Martin the Warrior was heavily fatigued, after having fought over a dozen evil vermin, WHO wished to attack the peaceable inhabitants of Redwall Abbey.”
However, most people would prefer to write “that” (or “which”, to some) rather than “who”, for the subject in question is not a human(s).

@Jean Your comment about the hyphens is both necessary and helpful; however, there is a simpler way to use an “M-dash”, as it is called: Simply hold down the “Alt” key, type in (on the number pad on the right of the keyboard) “0151″, and then release the “Alt” key. The result is called an “Alt code”, for those of you who are unaware. There are many more of them, though this one here is the only one needed for the discussion at hand.
Also, Jean, you misspelled “hyphen” the last time you typed it. Of course, we all know what you meant; but it is a misspelling nonetheless.

“just a reminder that “who” should always be used when referring to people.

“Who” should not always be used when referring to people. “Whom” is sometimes appropriate instead.

The example sentence:

- My car that is blue goes very fast.

is also artificial as no native speaker would ever say it. But even if it was, it’s not the use of ‘that’ or ‘which’ that (or which) makes the difference. It’s the use (or not) of commas, or in speech of pauses.

Jane Austen, for one, used both ‘which’ and ‘that’ in defining relative clauses.

As for the question as to the use of ‘whom’, most of us simply leave it out altogether, which we can do because it is the object:

- This the man (who/whom) I love.