Question

We had not even walked one kilometre when they stopped us.
(Begin the sentence with HARDLY

Answer 1

The verb "hardly" usually comes before a main verb. When you are writing, the word "hardly' can be used at the beginning of a sentence to say that one thing happens very soon after another thing:
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Hardly had we walked one kilometre when they stopped us. 

Answer 2

Hardly is an adverb that is used in various ways. We can also use scarcely or barely in place of hardly to say that one thing happens immediately after another: We had hardly/scarcely/barely driven a few meters, when the tyre went flat. In formal, written English, especially in a literary style, these words can be placed at the beginning of the sentence as shown in the following example: Hardly/Scarcely had we driven a few meters, when the tyre went flat. Note that we usually use when in these sentences, not than.

 

So the correct answer construction of the given sentence in the question is as follows:

Hardly had we walked one kilometer, when they stopped us.

Or

Scarcely had we walked one kilometer, when they stopped us.

Or

Barely had we walked one kilometer, when they stopped us.