Is leaving forums considered shunning members, falling under the Prophet's (PBUH) prohibition: "It is not permissible for a Muslim to shun his brother for more than three nights"?

Your departure from the forum does not fall under the blameworthy and forbidden abandonment (hajr), because it is not considered abandonment of its members, especially if the reason for it is contention over worldly matters. However, if there are religious violations such as mixing and forbidden conversations between men and women, then one must leave it, and it is not permissible to communicate with men thereafter, as it is a great gateway to fitna (temptation/strife). And you should advise your female friends to fear Allah.