What do you do with an overbearing mother?

My mother is overbearing when comes to her opinion on my life with my children.Yes, everyone has one, but when is enough a enough? Example I have three boys and my mother constantly trying to give me advice even when I don't ask on how to raise them. It starting to annoy me because my oldest is always "Grandma said this and that" and tries to play me against his grandma, his dad, and me. She tends to undermine my authority with my sons. I'm her youngest child and was the first one to move out at 19 because me and her butt heads and my children are her only grandchildren to give you a little insight. How do handle a overbearing mother? It's like she want to be their mother. I've already told her if any of my sons get out line and she had something to do with I would sign over custody to her because if he's only going to go by with what she say's then she needs to support him.

Here's little more information: No, I've never been a teenage mom, I use to babysit kids and work in daycare from age 11-19 during my younger years and I'm not the only relative she does this to, she's just worse with me. I tried to talk to her, but it tends to be a big argument.