Just quote scripture to them.
Memorize these very words of Jesus.
You will be confronted with this if you volunteer or work in a parish. Certain people will complain that money should not be spent on Jesus. That instead, it should be given to the poor.
"Now when Jesus was in Bethany, in the house of Simon the leper, a woman came up to him with an alabaster jar of precious ointment, and she poured it on his head, as he reclined at table. But when the disciples saw this, they were indignant, and said, 'To what purpose is this waste? for this might have been sold for much and given to the poor.' But Jesus, perceiving it, said to them, 'Why do you trouble the woman? She has done me a good turn. For the poor you have always with you, but you do not always have me. For in pouring this ointment on my body, she has done it for my burial. Amen I say to you, wherever in the whole world this gospel is preached, this also that she has done shall be told in memory of her.'"