Two issues here:
If you are disciplining him, and he's throwing tantrums and you decide not to buy him anything, you are absolutely in the right, and she has literally NO business in countermanding you. You can stand firm with her on this and insist that if you have said no, then No means No. If she wishes to take offence at your disciplining your own son, her problem.
If she is wanting to buy him toys and you are not wanting to give them to him until Christmas, then just tell her, we'll save them for Christmas. Again, your child, your rules.
She can get as upset as you like, but it's ultimately and utterly your decision.
As long as you hold onto that thought, you can calmly explain and re-explain if need be, that these are the decisions you have made for the benefit of your son.
Agree with Katisha in the short run, but that won't work even next Christmas, so it's important to nit this in the bud, gently but firmly sooner rather than later.
Inlaws are just like children, you have to be firm and consistent.