Unsubscribe Positions.Closed Event

Once an event has been added to be triggered when a position is closed, how can the event be removed?
Even after I use:
Positions.Closed -= OnPositionsClosed; 
the event still fires.

The example below demonstrates the problem.  Items closed by this line should NOT be reopened!
foreach (var position in Positions) ClosePosition(position);

using System;
using cAlgo.API;
using cAlgo.API.Internals;

namespace cAlgo.Robots
{
    [Robot(TimeZone = TimeZones.UTC, AccessRights = AccessRights.None)]
    public class MoneySalad : Robot
    {
        int ii = 0;
        protected override void OnStart()
        {
            Positions.Closed += OnPositionsClosed;
            ExecuteMarketOrder(TradeType.Buy, Symbol, Symbol.NormalizeVolume(100000, RoundingMode.ToNearest), "Initial", null, null, null, "Initial");
            ExecuteMarketOrder(TradeType.Buy, Symbol, Symbol.NormalizeVolume(100000, RoundingMode.ToNearest), "Initial", null, null, null, "Initial");
            ExecuteMarketOrder(TradeType.Buy, Symbol, Symbol.NormalizeVolume(100000, RoundingMode.ToNearest), "Initial", null, null, null, "Initial");
        }

        protected override void OnTick()
        {
            if (ii++ > 30)
            {
                ii = 0;
                Positions.Closed -= OnPositionsClosed;
                foreach (var position in Positions) ClosePosition(position);
                Positions.Closed += OnPositionsClosed;
            }
        }
        private void OnPositionsClosed(PositionClosedEventArgs args)
        {
            var position = args.Position;
            ExecuteMarketOrder(position.TradeType, MarketData.GetSymbol(position.SymbolCode), 100000, "Mart_" + position.Id, null, null, null, "Mart_" + position.Id);

        }
    }
}