As long as it is not un-safe for your client to talk, let him/her talk. It will ensure they are breathing, rather than holding his/her breath.
If they are getting out of breath, I would recommend (although you may have tried this already) you stop them. A Firm, “stop conversing for a moment, please.” and use it as a test – not a bad idea anyway.
“Let’s have you stop talking for a moment and see how quickly you recover, this will give us an indication of your fitness level.” “OK, let us take a moment to concentrate on tempo and breathing; when to breath in, when to exhale to get the most of your workout”
You will need to allow them to go back into their pattern, but if you get them to disengage from talking regularly, it will be a new part of your session.
Finally, the last piece of advice I have is to come to your sessions with talk topics of your own. If they are a talker, it usually doesn’t matter what the topic, so you might as well enjoy the exchange.