You do know they can't stop you if you make it clear it's a parody, right? That's the law. Parody and satire are considered First Amendment freedom of expression and protected speech. What you need to do is make sure the name reflects the parody status and include a disclaimer. You have to include in the disclaimer that the work is a parody with no affiliation with or authorization from Marvel or Disney. You'll probably also need to state that you are not making any profit, just to be on the safe side and don't charge for anything. Don't use any assets of theirs, meaning no directly copying artwork and no copyrighted music cuz that will get you into trouble.
I'd say take a cue from upthread and titled it "Here Cums the Spiderman, a Porn Parody"
Do that and you should be fine. They may send you another letter anyhow, but they know they can't touch a clearly labeled parody. The settled case law is very clear on that.