What is the ruling on my warning a person who used my private materials for an independent venture, and is his act forbidden, and is it permissible to sue him and demand my rights?

There is no harm in warning someone against using your personal belongings. He would be committing a sin by using your tools despite your warning, for the Prophet said: "The property of a Muslim is not lawful except with his willing consent." You have the right to demand your dues from him. If he does not give them to you, you may sue him. If you forgo demanding your dues due to his difficult circumstances, we hope you will be rewarded for it.