How to I ask my in-laws to stop spoiling my child?

Nothing goes wrong with communicating well with them. Tell them how you feel and make sure to set guidelines but do so in a way that they won't feel offended or disrespected I totally agree with your battles about spoiling, but then again there's nothing wrong with your in-laws giving them things from time to time, but there is a certain extent for them to give. Just consider letting them know of your concerns, I'm sure it'll turn out well!

Sit down with your in-laws together with your husband and politely tell them that you want to grow your child in your own way. Mention all the things that you don't want them to implement to your kids.

I think it's my wife's job to tell it to her parents so as there wouldn't be miscommunication and other discussions. It works well with us, maybe you should give it a try.

Politely tell to your in-laws that you don't like the act of splurging stuff for your kid. Make sure that you don't sound too harsh, though.