I did a similar to my daughter once.She had a habit of throwing dirty clothes under the bed.
I told her when buying her new school clothes, "I had better not fid these under the bed".
A week later there they were.
I took everything she had except what whe was wearing and hid them.
She had to wear the same outfit to school for 2 weeks before I gave her stuff back.
She would wash them every night (afraid to let me do them for fear I would keep them too).
A month later all was forgotten and clothes under the bed again.
I just gave up.
J J