These are some polite ways you could ask the interviewee to do what you want:
Please stay on topic.
Please answer the question I gave you.
You're deviating from the question.
You're straying from the main point – let's get back to the question.
We're getting sidetracked here, can we get back to the question I asked originally?
That's a nice aside, but how does all that relate to my question?
If you want a more hardline response that will make the interviewee more uncomfortable, you might try something like one of these:
That's all well and good, but let me repeat the question, and let's see if you can answer it this time.
That's amazing. I just listened to you ramble on for five minutes and I don't think you came close to answering my question. Are you even listening to me?
Words I've put in bold are key words and phrases that might help you addess the situation in very natural-sounding English. The phrases I've put in italics denote a measure of sarcasm, which is why I've put them in the second set of responses.