Left-arm spinners Veerasammy Permaul and Jomel Warrican combined for nine Sri Lanka wickets with career-best performances.
According to an ICC statement, Kieron Pollard pleaded guilty to the offence and accepted the proposed sanction, so there was no need for a formal hearing.
Dasun Shanaka praised his team for a complete performance that help them beat defending champions West Indies by 20 runs.
T20 World Cup: West Indies captain Kieron Pollard takes responsibility, says senior players didn't deliver
He had no hesitation in admitting that they were poor in all departments.
Nissanka played some outrageous strokes en route his third half-century of the tournament.