A slugfest between Wall Street and Main Street took an unexpected turn late on Wednesday after moderators of a stock trading forum that has helped fuel massive rallies in the shares of GameStop temporarily closed its doors. Shares of GameStop and other companies tumbled in extended trading after Wallstreetbets, a discussion forum popular with retail traders on the Reddit website, briefly turned invitation-only. They pared those losses around an hour later, when the forum opened back up. “We have grown to the kind of size we only dreamed of in the time it takes to get a bad night’s sleep. We’ve got so many comments and submissions that we can’t possibly even read them all, let alone act on them as moderators,” read a message from the group’s moderators after Wallstreetbets reopened. Shares of GameStop, AMC Entertainment, Koss Corp and BlackBerry all dropped at least 20 percent moments after the …