Does anyone know of any strict, free, computer filters that I can use to help me on this NoFap journey?
The good ones are going to be an investment unfortunately. There are many free ones available but they all have weaknesses. In this case, the cost is worth what you get in return. I use Norton and it's never let me down. I'm not trying to bypass my filters because I genuinely never want them to fail. But it had the best reviews of any that I've researched.
Google K9 filter; works just about as well as any product you pay for and in my experience, no different.