I think I'm gonna tell my mom not to buy gifts for a struggling family this year.
Why don't you want your mom to buy them presents?
I don't want them to have a good Christmas.
You don't have the right to tell your mom how she should spend her money. If she wants to help another family out and buy presents, you need to let her do it.
I'm going to stop her any way I can.
I don't understand your rationale, but you have no place telling your mom what to do with her money.