If what the OP asked for is clearly impossible, it is acceptable to state so in a full answer, as long as a reasonable explanation is provided.
If you just want to say that it's impossible, it's not worthy of an answer as it is little more than noise. At Stack Exchange, we want to keep the quality of answers high, so an answer that states that the desired action is impossible needs to be backed by an explanation.
For example, let's say someone asked the question, "What is 4 divided by 0?"
The answer I would give would be like this:
You cannot divide by zero. It is considered undefined.
Division by zero is undefined because you are effectively trying to solve for x in the equation x ∙ 0 = 4. As there is no value x that can satisfy this equation, it is not possible to define division by zero (given a non-zero dividend). In the case of zero divided by zero, any value of x will satisfy x ∙ 0 = 0, making this operation indeterminate and therefore undefined as well.
For more information on division by zero, see this Wikipedia article.
