Is the owner of the website entitled to expel one of its founding members after having caused her distress and made her quit, especially given that she was one of the first to join the website?

A woman is permitted to participate in general forums, subject to certain controls, including avoiding private messaging with men due to the potential for fitna (temptation/discord). It is advisable to leave the forum, cut ties with the man, and not regret losing one's status, for whoever abandons something for the sake of Allah, Allah will compensate them with something better. A woman can participate in forums by writing and offering advice without being involved in the administration, or she can cooperate with righteous women in a women's forum.