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I've been learning Japanese for a few days and there is one thing that I want to know:
How is "forbids" expressed on signs in Japan? And how in formal texts?
I suggest that the form "[x]は きんし です。" is more colloquial and that in signs and in formal text there is the form "いけません" (= it is forbidden to...).
You can, if you want, make examples (e.g. for signs: Smoking forbidden ...; or formal texts...)
Even though I am not entirely sure what exactly you are trying to ask, I will somehow manage to talk about the things that I feel might be of interest to Japanesse-learners.
With Japanese signs --- any signs really, including those asking you not to do something --- things happen that do not happen with signs in other languages. That is regarding how the message should be written. Specifically, which ones of the three writing systems we should use.
For instance, kids under 10 or so could not read the following sign at the beach prohibiting swimming. It says 「[遊泳禁止]{ゆうえいきんし} [石巻市]{いしのまきし}」 meaning "Swimming Prohibited. The City of Ishinomaki"
You will often find another sign nearby saying something like 「ここでおよいではいけません」 = "You cannot swim here." in kana so that smaller kids could read it. What you will NOT find, however, is a sign that says 「ゆうえいきんし」 because that still requires a ceratin level of vocabulary to understand despite the use of no kanji.
By far the most often-used phrase on signs to keep kids away is:
「よいこはここであそばない」 = "Good kids do not play here."
禁止{きんし} can actually frequently be found on Japanese signs. 喫煙禁止{きつえんきんし} - "No smoking" and 立入禁止{たちいりきんし} - "Do not enter" are common examples. Due to the concise nature of this word (only two characters), it actually lends itself quite nicely to prohibition signs. A Google image search of 禁止 will actually yield nothing but prohibition signs.
As for expressing prohibition in a sentence, you are right in assuming that いけません could be used. I'm still learning myself, but I'm fairly sure that it's the most common way (of course including the variants いけない、ならない、etc.) - I cannot comment on the usage of 禁止{きんし} to prohibit something in full sentences, but my hunch is that it would sound very stiff and formal and - if it is used at all - is not an option you should consider in general.
