There are two ways this can be done: by adding a meta tag in the page generated by AskiaExt, or with a robots.txt file at the root of the web server.
The latter is less fool-proof (the file can be deleted or corrupted), but it has the benefit that clients could allow search engines access if they want to (by changing file).

Example of a meta tag:

<meta name="robots" content="noindex,nofollow" />

Example of a robots.txt file:

User-agent: *
2Disallow: /cgi-bin/