1. Realize that when you use a popular or copyright character, you inherit the fan-base of that character.
Some fans are bat-shit crazy. Don’t be surprised when you get some weird request or questions. They might want to see that character in all kind of sexual acts and positions.
2. When people ask you questions they are not intentionally malicious or trying to rush you.
When is it going to come out ? This does not mean “hurry up”. They just genuinely want to know. Post often what you are currently working on in a blog or something so people can find information on what you are working on. People can’t read your brain or know what you are doing. They are asking because they are genuinely interested.
3. Realize the popular copyright character has TONS of fans and that you are the only (or one of the few) sources of hentai for that character.
Major developers and publishers in main stream media would not be making porn of their characters and franchises. You are the only one doing it. so people that have those similar fantasies will come to you to fulfill them with requests, commissions and all kind of weird fetishes. Some will even harass you about it. rather then ingnoring these people, communication is they key, which goes into number 4 below.
4. Don’t Isolate yourself or stop replying to messages
Make clear what you are or are not willing to do in plain text that people will see to lessen questions and messages and requests. You will always get people asking you to do stuff. It will not go away but you can lessen it. You will discourage yourself and loose motivation if you go into hiding. They may be annoying as hell but it is someone who is genuinely interested in your project and/or your art ability. Also don’t ignore people, if there is something you don’t feel uncomfortable doing, then say it; reply to them clearly. If you are not clear they might misunderstand and make the request again or harass you about it if they still think they have a chance of changing your mind.