I want my mother to abstain from buying christmas gifts this year
Why do you want your mother to not buy any Chrismas gifts?
A christmas gift means a lot for my mom.
It probably means a lot to your mother to gift things. It is not polite to tell someone you do not want a gift from them.
I don't think she can afford it.
Your mom is allowed to do what she likes with her money. If you want something simple you could give her hints or tell her about it. She might make a note and then give you something like that.