A counter-troll is someone versed in the art of disarming an online troll, usually by out-witting, out-exasperating, or purposefully failing to be antagonized by the troll's actions.
These actions are different from feeding the troll as the latter involves giving the troll ammunition and sating its hunger for chaos and bedlam, whereas counter-trolling involves responding to the troll in a manner that leaves him/her with nowhere to go in terms of antagonistic actions, and will likely deter him/her from further trolling.
Note: Counter-Trolling must be done very carefully, as a desperate troll won't understand what's going on and resort to general pandemonium, resulting in combustion, turning the troll into a flamer.
TROLL: Counter-trolls are gay.
TROLL: That means you like it up the ass.
TROLL: That means you have a boyfriend you faggot.
TROLL: Fuck you.
*TROLL has signed off*
COUNTER-TROLL: Awesome, now I can wrap up my activity here in peace and quiet, go home, and sleep with my wife.