Would you say that Destroy Magic* can be used on the turn just before someone enters the Valley of Fire** to force them to turn back?

*Cast at the start or end of your turn. No character, including yourself, may cast a Spell or use any Magic Object until the start of your next turn.
**You can only enter if you have a Talisman. If you do not have one, you must turn back.

I thought this was a given anyway? is there a FAQ on it though?

Yes, there is a FAQ entry that officially ended this once good use of Destroy Magic.

If you take the words literally and stop thinking thematically, the FAQ answer is the only possible one. You must have a Talisman to enter the Valley of Fire, you needn't use it. There's a specific paragraph in the rulebook that explains the difference:

Having and Using Cards
Characters are considered to have anything in their possession, such as Objects, gold, fate, Followers, and Spells.

When a character implements the ability of a card’s text box, he is considered to be using the card. Using cards is optional, and a character may always choose when to use a card he has. For example, the Cross allows a character to automatically destroy Spirits without resorting to psychic combat. The character may choose not to use the Cross and may attack a given Spirit instead.

The Talisman card text is just a reminder, not an ability that requires to be used. According to the Valley of Fire instructions, it's enough to have a Talisman to enter.

Well I guess I should have known that one without asking, but it would have been a great use for an otherwise pretty useless spell.

Well I guess I know that one without asking, it would have been a great use for an otherwise pretty useless spell imo.

It used to be for a long time, now it has been sacrificed for rules' consistency.