Is it obligatory for me to refuse the use of my devices for haram, or to refuse charging others' devices with my charger, or to refuse buying mobile phones for them, and what is the proper way in these situations?

There is no sin upon you in offering the service, and you are not obliged to refrain from it. It is better, if you fear the use of mobile phones for impermissible purposes, that you take the initiative to advise them to use the blessings God has bestowed upon them in ways that do not involve disobedience to God. You should also strive to strengthen their faith by admonishing them or by gifting them audible and visual admonitions and advice.