Can I keep my MIL from seeing her granddaughter?

My husband and I agree about this. His mom using presription drugs to the point where she is high---she abuses them. She will call my husband and leave messages how she was a good mother (even tho she used heavy drugs his whole life), cry and yell thru the whole message. Part of the messsage is incomprehensible. My nephew and my SIL lived with MIL and my nephew at age 3 could say pills are bad. He saw her when she was abusing drugs all the time. Needless to say he knows way more than an 8yr old should know. I do not want that for my daughter and frankly I am tired of the drama. I refused to let her come in the other day. She called my other SIL complaining how I can't keep her from her granddaughter. Doesnt the parent get a say in who sees their child esp when they believe that person is a bad influence. What should we do? We don't want her around our daughter until she has been clean for a long long time. She will go maybe a month before she uses again, and she uses the sad excuse "Oh but I am sober." She will be for a quick minute, but I dont wanna risk her coming over when she is high, I am not gonna drug test her before she comes in my home so I ume she just stay away.