Is there any idiom or proverb discouraging knowledge?

I have only found "Ignorance is bliss", is there any other?

Answer

Not knowledge per se,
but the classic warning against the pursuit of knowledge
is curiosity killed the cat:

Wikipedia:

“Curiosity killed the cat” is a proverb used
to warn of the dangers of unnecessary investigation or experimentation. …

The Phrase Finder:

Inquisitiveness can lead one into dangerous situations.

The Free Dictionary:

Prov. Being curious can get you into trouble. 
(Often used to warn someone against prying into other’s affairs.)


• Jill: Where did you get all that money?
  Jane: Curiosity killed the cat.

the Cambridge English Dictionary:

said to warn someone not to ask too many questions about something

I could tell you, but then I would have to kill you.

Quora:

I could tell you, but then I’d have to kill you
has become a way to let someone know they’re asking you something
you don’t want to answer
(and a rather colorful way of telling them to mind their own business!).



This line comes from "The Hounds of Baskervilles" featuring Sherlock Holmes. 
Here is the exchange:









Sherlock: I never did ask, Dr. Franklyn. 
What is it exactly that you do here?
Doctor: Oh, Mr. Holmes, I would love to tell you,
but then, of course, I’d have to kill you.
Sherlock: That would be tremendously ambitious of you.








    ︙





The line has since appeared in many movies,
including Top Gun with Tom Cruise. 

Here’s a handy YouTube video
compiling many snippets with this line and variations on it.

The Free Dictionary:

a phrase said in answer to a question that one does not want to answer. 
Don’t ask.

TV Tropes:

Often heard in settings related to espionage and high security levels,
the phrase “I’d tell you, but then I’d have to kill you” is itself

probably a Dead Horse Trope by this point –
whether for a serious straight use or not.

As the quotes above indicate, this has become something of a joke.

While not exactly an idiom or proverb,
the phrase knew too much is evocative of the films
The Man Who Knew Too Much,
in which a man is murdered for learning of a criminal conspiracy. 

References: [IMDb 1934], [IMDb 1956], 
[Wikipedia 1934], [Wikipedia 1956].

There’s a well-known proverb, “A little learning is a dangerous thing,”
often misquoted as “A little knowledge is a dangerous thing.” 
For years I (mis)understood this to mean that knowledge is dangerous —
so dangerous, so potent, that it is dangerous even in small quantities. 
But when one sees the complete sentence, which appears in Part 2
of An Essay on Criticism by Alexander Pope:

A little learning is a dang’rous thing;
Drink deep, or taste not the Pierian spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again.

it becomes clear that the message is that partial or incomplete knowledge is dangerous, while thorough or complete knowledge is a good thing.

The Phrase Finder:

A small amount of knowledge can mislead people into thinking that they are more expert than they really are.

Dictionary.com:

Knowing a little about something tempts one to overestimate one’s abilities.

and from this we get the phrase (again, not exactly an idiom or proverb) knowing just enough to be dangerous.