What is the authenticity of the message that threatens whoever watches pornographic films and images with deprivation from Paradise, expulsion from God's mercy, a promise of Hellfire, and torment therein, and that whoever withholds a good deed will be bridled with a bridle of fire?

We do not know the origin of what was stated in that message. It is not permissible to terrify people about forbidden things with matters that have no basis, for this is a lie and a statement against Allah without knowledge. The established proofs of prohibition are sufficient. The concluding statement of the message, "Whoever conceals knowledge, Allah will bridle him on the Day of Resurrection with a bridle of fire," has been reported from the Prophet, peace and blessings be upon him, and it is a good and authentic hadith.