He runs a company hosting web domains and found out that a website owner was using it to lure women
Question

                I run a company for web domains. Two months after selling a site, I found out that its owner is using it to seduce girls and then drag them to haram. He confessed to me before that he had haram relationships through internet. 

I have sent da’wah materials to him, so that may Allah guide him, but he insists on what he is doing. 

I have closed the website and put da’wah materials instead of its previous content, and wrote a warning message about this man’s bad intention so that the sisters are careful.  

Now he is asking for what he paid me initially, my question is: do I have to return what he paid or not?.
Praise be to Allah.
    If the matter is as you describe, then you did well by 
    shutting down the site and advising those who go to it, and we ask Allaah to 
    reward you with good. 

    The basic principle is that contracts of hiring or renting 
    are binding on both parties until the end of the specified time period, but 
    if it becomes clear that the renter is renting the service for haraam 
    purposes, then this makes it permissible for you to annul the contract. 

    As for the payment agreed upon for hosting web domains, you 
    are entitled to a portion of it commensurate with the period during which he 
    used the site, and you should return the rest to him. 

    We put this question to Shaykh Dr. Khaalid al-Mushayqih (may 
    Allaah preserve him) and he replied: Yes, if what he was doing was haraam, 
    then you should annul the contract, because Allaah says (interpretation of 
    the meaning): 

    “Help you one another in Al‑Birr and At‑Taqwa (virtue, 
    righteousness and piety); but do not help one another in sin and 
    transgression”

    [al-Maa’idah 5:2]

    As for the rent that he paid, he is entitled to a portion of 
    it, i.e., if he had used up half of it, he gets half back, and so on. End 
    quote. 

    And Allaah knows best.