A: It is important that you and your husband recognize that your mother-in-law's choice to buy unnecessary toys is entirely up to her. She has her own reasons--which are probably joyfully innocent. Try pointing out again that her grandson loves her and that he already has plenty of toys. Tell her that you hope she doesn't feel that she has to keep buying him more, but this is about all that you can do.
As the parents, you do have the right to interfere if she buys your little guy a sharp pocketknife, takes him into an "adult" bookstore, or encourages him to lie. You cannot allow anyone do something dangerous to your son. But saving for his college education is not going to be less of a challenge if you prevent Grandma from loading him up now with new toys that do no harm. So I would relax and let go of the issue.