Completely blocking unwanted SMS text messages from random numbers

Hello everyone. Is there any apple or 3rd party application that can be used to completely block unwanted sms text messages from random numbers that aren’t in my contacts? There is some individual that is constantly texting my phone number at all times of day and night from various numbers. If I block a number, they change to a different number and continue to harass me. Changing my number is an option, but I don’t want to use it. I’ve tried using the iPhone unknown message filter and the number still pops up under the “contacts & sms” column. This dilemma has gone on for months and has become very nerve wrecking and stressful. I’ve contacted law enforcement and they informed me if no threat is going on, they cannot perform an investigation. Can someone please help me.