I don’t recall the precise word now, but I had an already-hyphenated word in a paragraph that had automatic hyphenation turned on, and the only way to prevent an extra hyphen being inserted was either to (a) turn off auto-hyphenation for the whole paragraph, or (b) create a do-not-hyphenate dictionary entry for that specific word to prevent it from being hyphenated. But I may in future want to allow that word to be hyphenated.
Why would you manual hyphen a word in any text editor? Hard manual hyphens restrict any reflow of text while editing. This is precise the reason why auto hyphenation is developed.
If you want to do it manual anyway use these:
To manually enter a hyphen directly in the document, click in the word where you want to add the hyphen, and then press Ctrl+Minus sign (-).
To insert a non-breaking (protected) hyphen directly in the document, click in the word that you want to hyphenate, and then press Shift+Ctrl+Minus sign(-).
To hide soft hyphens, choose Tools - Options’ - LibreOffice Writer - Formatting Aids, and then clear the Custom hyphens’ check box.
It was a compound word with a long second part, and it was that second part that was getting hyphenated automatically.
But I have just thought of something: it was text imported from a Web site, so perhaps the hyphen that “came with it” was not being recognized by LibreOffice as a hyphen. Is that possible?
No, that does not seem to be the problem. I’ve tried typing my own text in a paragraph with automatic hyphenation turned on.
Suppose I am writing about “temperature-sensitive components” with a regular hyphen/minus between “temperature” and “sensitive”: depending on where this phrase comes at the end of the line, LO Writer will happily add a soft hyphen after “tem”, after “temper”, after “tempera”, after “sen”, or after “sensi”. All of these are violations of many style guides and look ugly.
The difficult part in your question is the intent to eventually hyphen the designated word in the future. You already know how to remove a specific word from hyphenation (entry in an exception dictionary – your b way).
If you don’t fear spelling error in the target word, tell LO Writer that the word belongs in no known language: this will disable spelling check and hyphenation (this is suggested by the built-in help) because LO Writer won’t find hyphenation rules.
Either create a character style with Language set to None in Fonts tab, but this will prevent you from applying another character style such as Emphasis.
Or apply this attribute through direct formatting “above” the regular character style.
Though I advocate against manual formatting, I think this is an acceptable exception because it will seldom be used in a document and it is targeted on very specific words, provided you remember you intentionally used direct formatting.
If this answer helped you, please accept it by clicking the check mark 
to the left and, karma permitting, upvote it. If this resolves your problem, close the question, that will help other people with the same question.
I see that this problem was reported more than two years ago, but I don’t see any indication that it is being worked on:
I added my example, and it just got marked as a “me too” duplicate.
I disagree that the problem applies only to “very specific words”: it applies to any compound word.
What I meant with very specific words was: it is some kind of flag superimposed to words to workaround a LibO limitation/bug. The workaround must be light enough to not impede global workflow et be erase easily once the problem is fixed. I agree that every compound word must be marked and that may prove tedious.
You might want to file an enhancement request for this reasonable (no idea how widely useful though) idea, like adding an option in paragraph’s text flow settings to not hyphenate already hyphenated words.
@mikekaganski: You also commented on this thread. Isn’t there a connection? Doing it on the character level instead of for words would allow for fine-tuning.
No. The asked thing is a styling matter (“we prefer to not put automatic hyphens if a word already has a hyphen”). Using character properties and applying them selectively to prevent something from happening is like using enters to get to next page, or spaces to right-align text.
Yes (within reason, regarding everyday speech) to first sentence, objection concerning the second one. Applying a character style is styling in the technical sense I would like to use the word in. To get it act as expected the AutoHyphenation needs to learn to respect it.
Concerning the styling (first sentence usage) I might want to control it to more detail: Don’t AutoHyphenate “Chuppledeewicksome-Leemolopressleakoning” except with at least a distance of 8 from the explicit hyphen.