How to stop guests from opening spoilers?

From memory, forum users had to be logged in if they wanted to view the contents of spoilers.
It seems that this has changed (I don't know when) in our forum without any obvious cause and everyone who visits can now open spoilers.
As members use spoilers to post sensitive stuff that might be libels, it's important to bar guests from seeing these.

Our list of permissions covering Guests, Members etc. lists categories and topics but not features like Spoilers.
I've searched this site under Spoiler and Guest but haven't found anything that seems relevant.

Has anyone else come across this problem of Spoilers which can be opened by all and sundry?

Any help and advice would be very welcome.

Hello,

Spoilers have always been able to be opened by guests, if a guest can read/see the post. If someone is posting sensitive information in a spoiler, that they don't want guests to read, they can put the spoiler in

Code:

[hide][/hide]

tags, then ONLY users who have posted in the topic and moderators/admins can see the spoiler and be able to open it. If guest posting is disabled, then they can't reply to the topic and as a result, the spoiler along with any content inside the hide tags, will remain hidden.

-Brandon