Removing Mold in Sink Drain?

Like a lot of others who have asked similar questions, I too have moldy and bad odors coming from my drains. When we shower or run the water, or let the water out, there is a bad odor, and moldy odors that travel throughout the house through the AC vents. I can see the mold on the inside of the PVC pipe in one side of the kitchen drain. I have tried bleach, and baking soda and white vinegar.

When I have mold start on the caulk on the side of the tub, I spray bleach on it, and it sets and gets rid of the mold. But how can I plug up the inside of the sink drain, so the bleach or whatever, can set right there on the mold to soak it and remove it. Anything poured down the drain just runs off and can't set on it and soak it. Is there something that I can put down the drain to temporarily plug it up in there, so the bleach can soak the mold loose off the inside of the drain? Something that could dissolve slowly. A plumber wants to sell us a $50.00 gallon of stuff, but how can it soak it if it just runs on down the drain? I wish I could get a small scrub brush down the drain to scrub he mold off. Thanks!