The fact that there were great tzadikim like Gra does provide an excuse for copycats. It is sadly all too easy to play on peoples naivety and credulity. And it is impossible to warn anyone about the frauds charismatic fakers because  of the prohibition of Lashon Hara (slander). Besides that, it is hard to tell the difference between miracles from the realm of holiness, and the exact same kinds of miracles from the realm of darkness. It is not worth the time to warn people. If someone of the stature of the Gra was ignored by his own disciples, then what good would my warnings do?