[shabble]

any attempt at accessible (that is, text based) content will make it wide open to automated solving and ruin the whole point.

Maybe an audio snippet could be used as an alternative, although actually creating the audio (unless it's done with text to speech methods at the server/creation time) is going to significantly bump up the cost of creating the advert.

[jnw2]

The Americans with Disabilities Act may not have much sympathy for the inconvenience of providing accessibility, though.