I'm thinking two things: either give her a clear ultimatum, or learn new ways to firm up your boundaries without getting drastic (since I'm guessing that your sons enjoy her visits). She sounds a bit like an honorary "uncle" of my grandson who really, really cannot help bringing toys and treats the parents do not want the child to have. But there's deep, genuine love on both sides, and it would be tragic to cut "Uncle" out of their lives.
To establish better boundaries, you might want to investigate a practical and effective process called Non-Violent Communication, which teaches a simple, 4-step method for listening and speaking compassionately, from the heart, so the discussion doesn't sweep either of you into negative emotions.
It will help you make clear and factual observations that will support your MIL's ability to hear what YOU need, without feeling attacked or overwhelmed and becoming defensive. And you can help her explore her own needs in a way that may give her new understanding about why she insists on doing things her way.
While it does take a bit of learning and practice, Non-Violent Communication is worth the effort, and can be effective even when used by only one person in a relationship. It gives you a mastery of effective communication and emotional "skills" that will serve you well in many relationships, whether personal or professional.
If this interests you, you can google the term for books, classes, summaries and examples. My husband and I have both learned these techniques and find them wonderful for understanding others, and even ourselves, better, and finding peaceful ways to resolve even sticky problems.
By the way, I spend most Fridays with my grandson, and always bring my Granny Bag, which contains materials for making art, educational stuff like maps and science experiments, and yes, the occasional toy that remains MINE (I buy toys that the kid in me still wants to play with!). But I take them home with me each time. They stay special and "new," and I rotate them regularly. Would your MIL accept such an idea?