Rule: No modifying a subject complement is emphatic.
Examples of errors:
(1) A woman may feel obliged to marry a man if she becomes pregnant by him. Later she may find out it was no good idea to marry him.
(2) I'll tell you what I know about Fasching, but I don't know very much about it, because I am no Catholic.
(3) I can tell you about some main difficulties as to pronunciation. There are for instance homographs which are no homophones.
(4) Lots of people demonstrate against things like nuclear power plants or obligatory military service. These people are no radicals or extremists, because they have thought about what they are going to do during such a demonstration.
(5) There are people who are no doctors that work in the evening too.
As already mentioned, many constructions consisting of a negative verb and non-assertive forms have alternative constructions with a positive verb and anegative word, e.g.:
non-assertive
I didn't see anyone.
He doesn't have any money.
That didn't do any harm.
negative
I saw no one.
He has no money
That did no harm.
These alternative constructions are synonymous, except that the version with the negative word is more formal in tone. Neither construction is usual in the case of be + subject complement, however. When no does occur before a subject complement, it has an emphatic or contrastive sense. There is a difference between
I'm not (a)Catholic
and
I'm no Catholic!
The second sentence is emphatic, having the effect of an emotional denial rather than a mere negative statement, and therefore almost demands emphatic intonation. The strong degree of emphasis inherent in this construction explains why it is often used in litotes--a rhetorical device of understatement whereby an affirmative statement is made more forceful by negating its opposite! Thus instead of saying
That's a major achievement.
one can say more emphatically
That's no minor achievement!
In any case, no + subject complement implies an emphatic denial, a denial to the contrary, of the underlying assertion. In (1), then, no good idea would imply the opposite of a good idea, just as no radicals... in (4) would be equivalent to
These people are certainly not radicals or extremists.
These are possible constructions, of course, but the point is that the special degree of emphasis and contrastive implication of no before a subject complement is not intended in any of the sentences above.