You sound a very caring person, I think it's sweet that you care so much not to hurt these people.

I have had slow fade done to me a few times and personally I think it's kindest, I didn't really mind and guessed what was happening. I also think when it's an online friend it's not as affecting as a real life friend, I mean online communication does tend to be more transient in nature in my experience.

Cutting off suddenly and abruptly I am less keen on, that can leave the person with thoughts they might have done or said something wrong rather than something just coming to a gradual phased out end.

Telling someone you don't have time anymore can be okay too (I have had that said to me too), again it's similar to slow fade out.

If you were my online friend I think I'd be touched that you thought enough of me to take the time to give consideration, and I'd understand.