That's really not hard to prevent. The easiest way is to blow out the flame.

Honestly, you couldn't burn enough candles inside the tent I have now to suffocate yourself. The front panel doesn't seal at all on the sides or floor so the air is constantly exchanging, hot air rising and pressurizing the air space near the ceiling of the tent where it escapes through the upper ends of the front panel and cold air rushing in below to replace it. Just your body heat causes that to happen.

Of course, the cold air is heavier and wants to stay near the floor so this creates the age old problem of your sleeping bag being on the floor of the tent where all the cold air is. That's the problem I'm attempting to deal with. I want to warm that air up down by the floor.

Another approach is to move the sleeping bag up where the warmer air is. A simple tarp over a hammock will work better with the heaters I'm playing with but I think controlling the way air flows can offer improvements to heating a tent with candles and I want to play with that some more yet.