This is a pretty common problem in my experience, and certainly was an issue for us.
Some of the issue stems from the fact that Grandma gets pleasure from giving the gift, and from the kid's reaction to getting the gift; asking her not to give them takes something away from her. It's a totally reasonable thing to do on your end - but it's important to understand that you're asking her to do something that makes her less happy.
What worked for us was talking with her, helping her understand our reasons for wanting to limit the gifts, and also making sure she understands we get her reasons for wanting to give them, and want to help her still be able to get what she needs out of the relationship while respecting our boundaries.
We talked with her about the issues with having too many toys - clutter, waste, our preferences on what they play with - and helped her understand that we weren't trying to take this away to hurt her, but instead had good reasons for wanting the limits we were setting.
Then, we worked out what was reasonable for us. First, she would have "her" toys, that she brought with her on visits, would play with them during visits, and then would take home. The kids really enjoyed them because they were new, or at least things they didn't see all the time. We enjoyed this because then they'd go away at the end of the visit and not clutter the house. (It helped that, with us, Grandma prefers to rent a hotel room or house and have the kids come to her, so there is some space there; this may not work as well when she's staying at your place.)
Second, we would tolerate a small thing or two, as long as it was within certain limits. A ball or something like that. But, it had to be something they could keep in a drawer, not a puzzle or a board game or whatnot.
Third, for "big" gifts, we talked ahead of time on those. We would explicitly give her links to Amazon or whatnot for the things they wanted; that way we ended up with exactly the right things.
As for enforcing boundaries, the main thing we did was, when this first started, any time she'd bring things with her, those were classified as "her toys" by default, and she had to take them back with her to put at her house. The kids understood we weren't "taking them away" from them exactly, but were making sure Grandma had them safe for them the next time she visited - so they didn't have any problems there.
Over time, this made it work pretty well - she could still give things to them, they just stayed with her, and she was okay with that. I'm sure it wasn't ideal from her point of view, but overall it worked out for all of us.
